ci: remove explicit gcc version for mpi ci

[mloubout]

gcc-9 not there anymore on gh runner, remove explicit version so it just picks whatever default is there

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.02%. Comparing base (892b78e) to head (33561e6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2787      +/-   ##
==========================================
+ Coverage   82.96%   83.02%   +0.05%     
==========================================
  Files         248      248              
  Lines       50804    50804              
  Branches     4479     4479              
==========================================
+ Hits        42152    42180      +28     
+ Misses       7879     7857      -22     
+ Partials      773      767       -6     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	68.53% <ø> (ø)
pytest-gpu-nvc-nvidiaX	69.06% <ø> (ø)

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