CI: switch macos runner to latest gcc version

[mloubout]

No description provided.

[codecov]

Codecov Report

Merging #2046 (ddaf357) into master (5481082) will decrease coverage by 9.60%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2046      +/-   ##
==========================================
- Coverage   87.87%   78.26%   -9.61%     
==========================================
  Files         222      222              
  Lines       37406    37329      -77     
  Branches     5636     4784     -852     
==========================================
- Hits        32870    29216    -3654     
- Misses       4010     7538    +3528     
- Partials      526      575      +49     

Impacted Files	Coverage Δ
tests/test_mpi.py	0.88% <0.00%> (-98.17%)	⬇️
devito/mpi/routines.py	29.05% <0.00%> (-66.47%)	⬇️
tests/test_data.py	39.09% <0.00%> (-58.79%)	⬇️
devito/data/utils.py	33.47% <0.00%> (-55.65%)	⬇️
devito/data/data.py	43.09% <0.00%> (-52.70%)	⬇️
devito/mpi/distributed.py	53.40% <0.00%> (-38.71%)	⬇️
devito/mpi/halo_scheme.py	74.75% <0.00%> (-22.78%)	⬇️
tests/conftest.py	60.50% <0.00%> (-22.30%)	⬇️
devito/types/sparse.py	66.10% <0.00%> (-21.86%)	⬇️
tests/test_builtins.py	78.96% <0.00%> (-21.04%)	⬇️
... and 58 more

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[FabioLuporini]

uncontroversial