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 85.91%. Comparing base (75f6635) to head (be2773e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   85.21%   85.91%   +0.69%     
==========================================
  Files          32       32              
  Lines        1867     1867              
==========================================
+ Hits         1591     1604      +13     
+ Misses        276      263      -13     

see 1 file with indirect coverage changes

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[fingolfin]

@cdwensley CI for this was failing, this should not have been merged

[cdwensley]

@fingolfin Sorry - now reverted. I have received a whole string of CI emails for various packages, and assumed that they all needed to be merged.

[fingolfin]

Sure but only if/when they pass. That's the entire point pf CI tests after all. Otherwise you might as well stop using them

[cdwensley]

My guess is that this PR should be reopened, but I cannot see how to do so.

[fingolfin]

Can't be reopened, I'll make a fresh one.