Delete stale branches

We have 5 [stale branches](https://github.com/apache/maven-compiler-plugin/branches/stale) having no commit in the last 2 years or more. Can we delete the following branches?

[] [MCOMPILER-320_sunmisc](https://github.com/apache/maven-compiler-plugin/compare/maven-compiler-plugin-3.x...MCOMPILER-320_sunmisc) (1 commit 6 years ago): I'm not really sure what this commit is about.
[] [MCOMPILER-320](https://github.com/apache/maven-compiler-plugin/compare/maven-compiler-plugin-3.x...MCOMPILER-320) (1 commit 6 years ago): add a new `additionalClasspathElements` configuration parameters, I presume in support of above commit. I presume that we don't want to encourage class-path specified that way.
[] [MCOMPILER-445](https://github.com/apache/maven-compiler-plugin/compare/maven-compiler-plugin-3.x...MCOMPILER-445) (1 commit 4 years ago): Add one integration test, but it is not clear what it wants to test (maybe javadoc warnings at compilation time, but it does not really test anything specific to the compiler plugin).
[] [ci-reporting](https://github.com/apache/maven-compiler-plugin/compare/maven-compiler-plugin-3.x...ci-reporting) (12 commits 2 years ago): I suspect that it was an experiment, maybe obsolete by now.
[x] [java11](https://github.com/apache/maven-compiler-plugin/compare/maven-compiler-plugin-3.x...java11) (1 commit 2 years ago): a nice attempt to reduce carbon footprint, but I think that it is included in #975.