Update CI to use setup-gap@v3

[fingolfin]

Update the CI workflow to use current gap-actions releases, migrate legacy inputs, and standardize the GAP version matrix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 23.37%. Comparing base (83f737d) to head (d50d37b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   26.14%   23.37%   -2.77%     
==========================================
  Files          14       14              
  Lines         876      860      -16     
==========================================
- Hits          229      201      -28     
- Misses        647      659      +12     

see 9 files with indirect coverage changes

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[olexandr-konovalov]

Thanks @fingolfin