#11674 Run only on macOS 12 with latest Python.

[adiroiban]

Scope and purpose

Fixes #11674

MacOS 10 will be removed soon.. or already removed.

Macos11 with Python 3.7 the minimum version fails with

File "/Users/runner/work/twisted/twisted/.tox/alldeps-withcov-posix/lib/python3.7/site-packages/twisted/internet/test/_posixifaces.py", line 31, in <module>
    libc = CDLL(find_library("c") or "")
  File "/Users/runner/hostedtoolcache/Python/3.7.14/x64/lib/python3.7/ctypes/__init__.py", line 364, in __init__
    self._handle = _dlopen(self._name, mode)
builtins.OSError: dlopen(, 6): no suitable image found.  Did find:

stop pretending that we care about macOS and just remove the CI for older versions.

[adiroiban]

all green . please review