I can't use a property for my project's parent version despite described as such in Spring Cloud doc - maven

Question

In Spring Cloud Config documentation, pom.xml's parent configuration is presented with a version variable spring-boot-docs-version as such 1:
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>{spring-boot-docs-version}</version>
<relativePath /> <!-- lookup parent from repository -->
</parent>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>{spring-cloud-version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-config</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
So I added the following in my pom.xml's properties:
<properties>
<spring-boot-docs-version>2.1.8.RELEASE</spring-boot-docs-version>
<spring-cloud-version>Greenwich.SR3</spring-cloud-version>
</properties>
But it doesn't seem to work; even by prefixing the {spring-boot-docs-version} expression with $ doesn't make maven resolve the parent POM.
Note however that I had to add this extra $ in the <dependencyManagement>:
<version>${spring-cloud-version}</version>
I searched the Spring Boot documentation too, but only found out it's not using a variable 2:
<!-- Inherit defaults from Spring Boot -->
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.1.8.RELEASE</version>
</parent>
How to use such a variable in these (external) parents' <version>tag?
Note: I also read that if you don't inherit from one of these starters, you'll loose some maven pre-configuration (like plugins & so.)