ci: Migrate macOS runners from macOS 12

[moreati]

The macOS 12 action runner is being removed, brownouts in Nov, death on 2024-12-04 https://github.com/actions/runner-images/issues/10721

[moreati]

Affects Azure DevOps as well as GitHub Actions

[moreati]

https://github.com/mitogen-hq/mitogen/actions/runs/11669080981/job/32490230504?pr=1176

TASK [Create /tmp/512mb.zero creates=/tmp/512mb.zero, _raw_params=dd if=/dev/zero of=/tmp/512mb.zero bs=1048576 count=512;
chmod go= /tmp/512mb.zero
] ***
Monday 04 November 2024  17:08:28 +0000 (0:00:00.007)       0:00:26.162 ******* 
changed: [target]
TASK [Fetch /tmp/512mb.zero src=/tmp/512mb.zero, dest=/tmp/fetch-{{ inventory_hostname }}-512mb.zero, flat=True] ***
Monday 04 November 2024  17:08:30 +0000 (0:00:01.338)       0:00:27.501 ******* 
ERROR! [mux  5271] 17:08:37.240824 E mitogen: <Stream unix_client.5683 #1dd0> crashed
Traceback (most recent call last):
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 3677, in _call
    func(self)
    ~~~~^^^^^^
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 1923, in on_transmit
    self.protocol.on_transmit(broker)
    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 2371, in on_transmit
    self._writer.on_transmit(broker)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 2111, in on_transmit
    written = self._protocol.stream.transmit_side.write(buf)
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 2237, in write
    written, disconnected = io_op(os.write, self.fd, s)
                            ~~~~~^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/runner/work/mitogen/mitogen/mitogen/core.py", line 595, in io_op
    return func(*args), None
           ~~~~^^^^^^^
BlockingIOError: [Errno 35] Resource temporarily unavailable