springboot Favor exec-maven-plugin shorthand property over redefinition

With this change, users can now rely on the standard exec.mainClass property when executing the exec-maven-plugin. Users of the start-class property are unaffected.

Thanks @hboutemy for the initial diagnosis and suggestion for the fix.

Comment From: fbiville

For more context, I am writing a helper for projectriff streaming processor. While the project uses Spring Boot, the helper is completely standalone and it took me a long time to understand that Spring Boot parent POM actually rewrites the exec.mainClass property, hence forcing me to comply.

While this issue is far from new, it seems it surprised a few more people, judging by the votes on StackOverflow.

Comment From: snicoll

Thank you for the additional context. We've discussed this one and decided to remove plugin management for the exec plugin altogether. I've created #20883