Rename Jdk9 build classes and scripts

[pjfanning]

The classes and scripts are still useful. They are used for running tests that only compile for newer JDKs like 21 and soon, maybe Java 25. The rest of the build is designed to support Java 17 as the min Java version. The name is a legacy from when we supported Java 8.

Most of the stuff https://github.com/search?q=repo%3Aapache%2Fpekko+jdk9&type=code

Maybe a name like latest-jdks instead of jdk9.