Update gfortran in CI

[andrewwinters5000]

Set the gfortran compiler to be gfortran-12 as gfortran-10 is no longer available on the latest Ubuntu runners (actions/runner-images#10636).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (8a587cd) to head (6a6c33d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
==========================================
- Coverage   94.55%   94.50%   -0.06%     
==========================================
  Files          31       32       +1     
  Lines        2810     2820      +10     
==========================================
+ Hits         2657     2665       +8     
- Misses        153      155       +2     

Flag	Coverage Δ
unittests	94.50% <ø> (-0.06%)	⬇️

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