[PATCH] ci: upgrade to using macos-13

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
    ci: upgrade to using macos-13
    
    GitHub announced in April that the macos-13 pool is available
    [https://github.blog/changelog/2023-04-24-github-actions-macos-13-is-now-available/],
    so let's switch. This might also stave off CI failures we experience
    over in GitGitGadget (e.g. here
    [https://github.com/gitgitgadget/git/actions/runs/6729366919/job/18290134547#step:3:56])
    and in Git for Windows (e.g. here
    [https://github.com/git-for-windows/git/actions/runs/6708618181/job/18252834721#step:3:57])
    where occasionally macos-12-xl runners seem to be co-opted to cope with
    macos-12 workload (and the former don't have Python2 in their PATH).

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1607%2Fdscho%2Fswitch-ci-to-macos-13-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1607/dscho/switch-ci-to-macos-13-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1607

 .github/workflows/main.yml | 6 +++---
 ci/lib.sh                  | 6 ++----
 2 files changed, 5 insertions(+), 7 deletions(-)