Modernize derived coordinates

[bouweandela]

Description

Closes #3056

TODO: figure out which CMIP6 data still needs a fix with the new iris version. Do this by making a list of esmvalcore.dataset.Datasets that previously errored and loading all of them.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.97%. Comparing base (e232092) to head (25cc1f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2999      +/-   ##
==========================================
- Coverage   96.15%   95.97%   -0.19%     
==========================================
  Files         270      270              
  Lines       15807    15787      -20     
==========================================
- Hits        15200    15152      -48     
- Misses        607      635      +28     

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