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 79.70%. Comparing base (18f6366) to head (5d94449).

❗ There is a different number of reports uploaded between BASE (18f6366) and HEAD (5d94449). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (18f6366)	HEAD (5d94449)
	6	1

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
- Coverage   86.56%   79.70%   -6.87%     
==========================================
  Files          22       22              
  Lines        1005     1005              
==========================================
- Hits          870      801      -69     
- Misses        135      204      +69     

see 7 files with indirect coverage changes

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[fingolfin]

The CI failure should be fixed by PR #153.