Problem: I have to to specify version number for dependencies when using spring-boot-starter-parent 2.0.x - maven

Question

NOTE: I did not write this pom.xml from scratch, this project was started by someone else.
I don't have to specify the version for some of the dependencies with a spring-boot-starter-parent <parent>. That is, if it's version 1.5.x. If I change the version number to 2.0.x, it starts to complain about missing version numbers for the dependencies.
I read somewhere that the spring-framework-bom dependency also would make sure that I did not have to specify version numbers, but commenting it out doesn't really make any difference (that is, everything works without it when version is set to 1.5.x). Obviously it's also supposed to be set to something else than "import", but I really don't know anything about this (I didn't add this myself)
This is the output I get from maven when running mvn dependency:tree:
[INFO] Scanning for projects...
[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[WARNING] 'dependencies.dependency.scope' for org.springframework:spring-framework-bom:pom must be one of [provided, compile, runtime, test, system] but is 'import'. # line 24, column 20
[ERROR] 'dependencies.dependency.version' for org.hibernate:hibernate-validator:jar is missing. # line 53, column 21
#
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR] The project no.numbershopper:numbershopper:0.1.3 (/Users/rune/src/server/pom.xml) has 1 error
[ERROR] 'dependencies.dependency.version' for org.hibernate:hibernate-validator:jar is missing. # line 53, column 21
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException
This is the pom.xml (I've only changed the version inside parent to 2.0.5.RELEASE):
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>no.numbershopper</groupId>
<artifactId>numbershopper</artifactId>
<version>0.1.3</version>
<!--<packaging>war</packaging>-->
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.0.5.RELEASE</version>
</parent>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-framework-bom</artifactId>
<version>4.3.5.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-thymeleaf</artifactId>
</dependency>
<dependency>
<groupId>org.thymeleaf.extras</groupId>
<artifactId>thymeleaf-extras-java8time</artifactId>
<version>3.0.1.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-mail</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-devtools</artifactId>
<optional>true</optional>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-validator</artifactId>
</dependency>
<dependency>
<groupId>javax.validation</groupId>
<artifactId>validation-api</artifactId>
<version>1.1.0.Final</version>
</dependency>
<!--<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.5</version>
</dependency>-->
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.3.1</version>
</dependency>
<!-- <dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-rest</artifactId>
</dependency> -->
<dependency>
<groupId>org.flywaydb</groupId>
<artifactId>flyway-core</artifactId>
<version>4.2.0</version>
</dependency>
<!-- JPA Data (We are going to use Repositories, Entities, Hibernate, etc...) -->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<!-- Use MySQL Connector-J -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-configuration-processor</artifactId>
<optional>true</optional>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-security</artifactId>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.6</version>
</dependency>
<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-text</artifactId>
<version>1.1</version>
</dependency>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-core</artifactId>
<version>4.2.2.RELEASE</version>
</dependency>
<dependency>
<groupId>org.xhtmlrenderer</groupId>
<artifactId>flying-saucer-pdf</artifactId>
<version>9.1.9</version>
</dependency>
<!--<dependency>
<groupId>com.mortennobel</groupId>
<artifactId>java-image-scaling</artifactId>
<version>0.8.6</version>
</dependency>-->
<dependency>
<groupId>org.imgscalr</groupId>
<artifactId>imgscalr-lib</artifactId>
<version>4.2</version>
</dependency>
<!-- https://mvnrepository.com/artifact/junit/junit -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
<!-- https://mvnrepository.com/artifact/org.springframework/spring-test -->
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-test</artifactId>
<!--<version>5.0.8.RELEASE</version>-->
<scope>test</scope>
</dependency>
</dependencies>
<properties>
<java.version>1.8</java.version>
<thymeleaf-extras-java8time.version>3.0.1.RELEASE</thymeleaf-extras-java8time.version>
<thymeleaf.version>3.0.2.RELEASE</thymeleaf.version>
<thymeleaf-layout-dialect.version>2.1.1</thymeleaf-layout-dialect.version>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-maven-plugin</artifactId>
</plugin>
<plugin>
<groupId>com.microsoft.azure</groupId>
<artifactId>azure-webapp-maven-plugin</artifactId>
<!-- Check latest version on Maven Central -->
<version>1.3.0</version>
<configuration>
<resourceGroup>maven-projects</resourceGroup>
<appName>${project.artifactId}</appName>
<region>westeurope</region>
<javaVersion>1.8</javaVersion>
<deploymentType>war</deploymentType>
</configuration>
</plugin>
</plugins>
<resources>
<resource>
<directory>src/main/resources</directory>
</resource>
</resources>
</build>
</project>

Answer 1

Spring Boot 1.5 uses Hibernate Validator 5 whereas Spring Boot 2 uses Hibernate Validator 6. One change in Hibernate Validator 6 is that it's now in a separate group named org.hibernate.validator rather than being in org.hibernate.
You need to update your Hibernate Validator dependency to reflect the new group ID:
<dependency>
<groupId>org.hibernate.validator</groupId>
<artifactId>hibernate-validator</artifactId>
</dependency>
Alternatively, you could remove the dependency altogether as it's part of spring-boot-starter-web which you also have a dependency upon.