Clean up branches

[kwin]

Currently there are the following non-feature branches:

• master
• maven-site-plugin-3.x

Both deviate quite a bit, and the last release was cut from the latter branch.
There was a discussion started in https://lists.apache.org/thread/o96n7r4f9k4gs9mo0rqyoy5xlcyt4802.

There is right now no need for maintaining multiple branches but it is currently unclear why certain commits in master are not contained in maven-site-plugin-3.x.

[kwin]

This was done and master branch afterwards reset and the obsolete branch maven-site-plugin-3.x removed. There is still a reference towards the old master branch in https://github.com/apache/maven-site-plugin/tree/tmp/master-20260403 and to the maven-site-plugin-3.x branch in https://github.com/apache/maven-site-plugin/tree/tmp/maven-site-plugin-3.x-20260403.