Remove macos-12 CI job

[tautschnig]

GitHub has deprecated the macos-12 image and will cease to support in in December '24. See actions/runner-images#10721.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.88%. Comparing base (83922b2) to head (743e9cb).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8482      +/-   ##
===========================================
+ Coverage    78.38%   78.88%   +0.49%     
===========================================
  Files         1727     1727              
  Lines       188685   198333    +9648     
  Branches     18475    18475              
===========================================
+ Hits        147900   156453    +8553     
- Misses       40785    41880    +1095     

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[tautschnig]

@peterschrammel Can you please remove the job from the list of required ones?

[peterschrammel]

@kroening may have removed it already.