Update MacOS image to macOS-12.

[1uc]

GHA will drop support for macOS-10.15. Hence, we update to macOS-12. See #704 for more discussion.

[codecov]

Codecov Report

Merging #709 (eaf5ab6) into master (ce97dd9) will increase coverage by 0.41%.
The diff coverage is 98.40%.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
+ Coverage   81.97%   82.38%   +0.41%     
==========================================
  Files          67       67              
  Lines        4383     4503     +120     
==========================================
+ Hits         3593     3710     +117     
- Misses        790      793       +3     

Impacted Files	Coverage Δ
include/highfive/bits/H5Slice_traits_misc.hpp	89.56% <75.00%> (-1.43%)	⬇️
include/highfive/bits/H5Attribute_misc.hpp	77.58% <100.00%> (+3.58%)	⬆️
include/highfive/bits/H5Converter_misc.hpp	85.84% <100.00%> (-0.27%)	⬇️
tests/unit/tests_high_five_base.cpp	99.65% <100.00%> (+0.02%)	⬆️

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