Remove macOS 12 CI build job from the actions workflow

[garybuhrmaster]

Per actions/runner-images#10721
macOS 12 will be removed from github action runners on
12/03/24.

Remove the CI build from the workflow

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[garybuhrmaster]

FWIW, if you look at the current CI runs, you will see a warning that mentions the macOS 12 removal, and the impending brownouts (with build failures) that will start occurring in November in advance of the full removal.

As a future improvement, macos 15 is now available as a runner image.

[jhoyt4]

agree - we should remove macOS 12. I'd hold off on macOS 15 until macports starts to cache built packages else we'll get constant timeouts from the runners at the 6 hour mark due to the runner compiling all the dependencies (qt5, mysql8 etc).