bump minimum requirements

[totaam]

As per https://endoflife.date/macos, only MacOS 11 (aka "Big Sur") is still supported.
So we should bump the SDK minimum requirements to 10.15 or 11.

[totaam]

New issues (because, of course there's more):

• libxml2 : when building for SDK version 11 , it complained that the target was not 12... so I made it 12 and then it failed again with a different error - very similar to dev-libs/libxml2-2.9.10-r4::gentoo_prefix fails to build with Python errors and solved in exactly the same way (by hand) by setting PYTHON_LIBS = -lpython3.10 - can we switch back to 11?
• libxslt : errors building the tests subdirectory (just skipped it by removing it from the Makefile), then almost the same error as libxml2 - solved the same way

Strangely enough, none of the other modules using Python have hit these errors, so it must be some autotools detection magic gone wrong in these two related packages.

Then much later:

• pyobjc-core -Werror,-Wunused-but-set-variable - patch tweaked in 1208fff + dcb4782
• gstreamer fails to find a valid "python-with-distutils" - looking at the meson log:

  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/Users/macos/gtk/inst/lib/python3.10/site-packages/py2app/__init__.py", line 35, in <module>
    __version__ = pkg_resources.require("py2app")[0].version
  File "/Users/macos/.new_local/share/virtualenvs/etc-WJx60enz/lib/python3.10/site-packages/pkg_resources/__init__.py", line 956, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/Users/macos/.new_local/share/virtualenvs/etc-WJx60enz/lib/python3.10/site-packages/pkg_resources/__init__.py", line 815, in resolve
    dist = self._resolve_dist(
  File "/Users/macos/.new_local/share/virtualenvs/etc-WJx60enz/lib/python3.10/site-packages/pkg_resources/__init__.py", line 856, in _resolve_dist
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'py2app' distribution was not found and is required by the application

Because it's trying to import py2app (and why?) from the bootstrap python copy in /Users/macos/.new_local/share/virtualenvs/* - which doesn't have it..
I have no idea how to make meson use our python (would it be safe?) or how to tell the virtualenv one not to try to import py2app.
Workaround is to make it use the one from our installation of python (dirty - but this works since they're the same python version!):

PYTHONPATH=$JHBUILD_PREFIX/lib/python3.10/site-packages meson ---prefix ...

The same meson problem occurs with all the gst-plugins-* modules.

[totaam]

We now require 12 for both x86_64 and arm64:

gtk-osx-build/jhbuildrc-custom

Line 3 in 1aa8aba

setup_sdk(target="12", sdk_version="12", architectures=["x86_64"])

The remaining issues are tracked in #27