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 54.39%. Comparing base (db70247) to head (73e27ac).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   55.48%   54.39%   -1.10%     
==========================================
  Files          25       25              
  Lines        1386     1355      -31     
==========================================
- Hits          769      737      -32     
- Misses        617      618       +1     

see 7 files with indirect coverage changes

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[kaashif]

I'm not sure why the build-pkg-docs step is failing.

[fingolfin]

I know the cause and will fix it when I am back at a computer