ci: upgrade to using macos-13

In April, GitHub announced that the `macos-13` pool is available:
https://github.blog/changelog/2023-04-24-github-actions-macos-13-is-now-available/.
It is only a matter of time until the `macos-12` pool is going away,
therefore we should switch now, without pressure of a looming deadline.

Since the `macos-13` runners no longer include Python2, we also drop
specifically testing with Python2 and switch uniformly to Python3, see
https://github.com/actions/runner-images/blob/HEAD/images/macos/macos-13-Readme.md
for details about the software available on the `macos-13` pool's
runners.

Also, on macOS 13, Homebrew seems to install a `gcc@9` package that no
longer comes with a regular `unistd.h` (there seems only to be a
`ssp/unistd.h`), and hence builds would fail with:

    In file included from base85.c:1:
    git-compat-util.h:223:10: fatal error: unistd.h: No such file or directory
      223 | #include <unistd.h>
          |          ^~~~~~~~~~
    compilation terminated.

The reason why we install GCC v9.x explicitly is historical, and back in
the days it was because it was the _newest_ version available via
Homebrew: 176441bfb58 (ci: build Git with GCC 9 in the 'osx-gcc' build
job, 2019-11-27).

To reinstate the spirit of that commit _and_ to fix that build failure,
let's switch to the now-newest GCC version: v13.x.

Backported-from: 682a868f67 (ci: upgrade to using macos-13, 2023-11-03)
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

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