Add Downgrade CI

[fhagemann]

The Downgrade CI will check if some package versions are too old and should be dropped.
I used some time to clean up the [compat] section and adjust the lower bounds to the lowest versions still supported.

This PR is based on the current dev, but it can also be merged after merging #59 and redirected to main then.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.77%. Comparing base (664e8fe) to head (1069ab9).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   33.77%   33.77%           
=======================================
  Files          28       28           
  Lines        1874     1874           
=======================================
  Hits          633      633           
  Misses       1241     1241           

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