This keeps only the macos-14 jobs, which run on Apple Silicon M1,
and removes the macos-13 jobs, which ran on x86-64.

Other operating systems jobs continue to run on x86-64 machines
(and none on ARM, yet). Only the macOS jobs are removed.

This change leaves Python 3.7 without any macOS test job. That is
probably okay, since it has been end-of-life for some time, and
it remains tested on Ubuntu and Windows.

Currently they are the same. The macos-latest label will move to
later versions automatically in the future, like the ubuntu-latest
and windows-latest labels that we are already using.

In this repo, the macos-14 label had been used originally because
it was added before the migration of macos-latest to be macos-14
was completed. See github/roadmap#926. It
was kept for clarity of constrast with the macos-13 jobs that were
also in use, some for the same Python versions.

Now that the macos-13 jobs have been removed in c2bbaf4, the
macos-latest label can be used here without confusion.