Refactor and clean up ci

This is a summary ticket as we have an MR open that fixes a number of things.

• CC7 no longer has an image available and should be removed.
• The naming of the jobs and stages can be improved to be more consistent: #798 (closed)
• The release jobs can be further improved: #799 (closed)

Any references left to xrootd4 and/or eos4 should be removed as