Cleanup repositories

[ascheman]

Some projects are (already) archived or even no longer used by the Apache team.
Therefore, they are not needed when checking out the Maven sources.
Instead we add https://github.com/codehaus-plexus/.github as part of the sources (as it contains common GitHub actions for Plexus components).

This change is a late result of support-and-care/maven-support-and-care#77.

[hboutemy]

I tried to do a review: each commit added so much to discuss that I don't want to start reviewing the full PR

please create one PR per change (that should even be more granular than the 3 commits), because each change nature needs some discussion: many small discussions on many small changes will be much easier than one global one

[ascheman]

I tried to do a review: each commit added so much to discuss that I don't want to start reviewing the full PR

please create one PR per change (that should even be more granular than the 3 commits), because each change nature needs some discussion: many small discussions on many small changes will be much easier than one global one

Thanks for the feedback, I have now raised #21, #22, and #23 to replace this mostly.
I would like to see a little bit more alphabetical ordering in some files (to make alignment in the future easier), but postpone this to a later point in time.