Reapply "Update CI to use setup-gap@v3"

[fingolfin]

This reverts commit 9a1dabb / relands PR #95.

Should not be merged until CI passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.91%. Comparing base (c89de00) to head (fe8db8e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
+ 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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[cdwensley]

Currently PackageInfo.g has GAP >= 4.12 and the minimal test is failing.
Try 4.13 and see what happens?

[fingolfin]

No, I think the problem is flaky tests. I am looking into a more permanent solution right now...