Fix and modernize CI

[Robbepop]

• Adds proper, modern CI caching
• Fixed audit, coverage and outdated CI jobs

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.44%. Comparing base (07c468f) to head (d93d476).
Report is 2 commits behind head on master.

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Additional details and impacted files

@@             Coverage Diff             @@
##           master      #81       +/-   ##
===========================================
- Coverage   73.29%   60.44%   -12.85%     
===========================================
  Files          13       11        -2     
  Lines        1112      498      -614     
===========================================
- Hits          815      301      -514     
+ Misses        297      197      -100     

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[Robbepop]

I will fix the outdated CI job in another PR.