springboot Invalid variable reference in maven-shade-plugin configuration

This should fix my prior PR #20052. I am pretty sure this is correct, but sadly I seem to be unable to fully build the project to verify for sure. Might need a couple eyes from build masters to ensure. I am so sorry for the bad fix in #20052.

History: 1) after the Gradle migration it was incorrectly left as ${revision} 2) I blindly used Maven conventions to set it to ${project.version} 3) After trying to 2.3.0.BUILD-SNAPSHOT build with my change I discovered ${project.version} always resolves to 0.0.1-SNAPSHOT. 4) Here is where I think I found the rest of the Gradle setup uses ${version} for the current SpringBoot build version. So I used that. :)

In order to know for sure a simple child project inheriting from the spring-boot-starter-parent 2.3.0.BUILD-SNAPSHOT must use the shade plugin like so:

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-shade-plugin</artifactId>
            <configuration>
                <createDependencyReducedPom>false</createDependencyReducedPom>
            </configuration>
            <executions>
                <execution>
                    <phase>package</phase>
                    <goals>
                        <goal>shade</goal>
                    </goals>
                </execution>
            </executions>
        </plugin>
    </plugins>
</build>

Then simply execute ./mvnw package to see the error.

Comment From: snicoll

@scheerer thanks for letting us know about the issue. I took the liberty of applying Andy's suggestion myself as we are releasing M2 today and I'd like to make sure the starter has a valid POM.