CI: Update codecov action

[danielhollas]

codecov-action@v4 made the CODECOV_TOKEN mandatory for PRs from the origin (and pushes to the origin). This token needs to be generated on codecov.io and added as a repository secret.

[sphuber]

I don't see the codecov action in the checks above. I take it that is because there doesn't actually seem to be a CODECOV_TOKEN secret for this repo. Or is there more that would need to be done? I am personally not really a fan of it as historically it has mostly caused more problems than I think it solves. But I wouldn't be opposed to add it again and give a it a go to see how things go.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.51%. Comparing base (ef60b66) to head (580eb8d).
Report is 112 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6420      +/-   ##
==========================================
- Coverage   77.51%   77.51%   -0.00%     
==========================================
  Files         560      560              
  Lines       41444    41454      +10     
==========================================
+ Hits        32120    32127       +7     
- Misses       9324     9327       +3     

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[sphuber]

Thanks @danielhollas