Update CI to use setup-gap@v3, add missing dependencies

[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 74.56%. Comparing base (a361822) to head (889c2cf).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   74.38%   74.56%   +0.17%     
==========================================
  Files           7        7              
  Lines         851      853       +2     
  Branches       14       15       +1     
==========================================
+ Hits          633      636       +3     
+ Misses        218      216       -2     
- Partials        0        1       +1     

Files with missing lines	Coverage Δ
gap/NautyGraph.gd	100.00% <100.00%> (ø)
gap/NautyGraph.gi	81.77% <100.00%> (ø)

... and 1 file with indirect coverage changes

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