refactor src/_pytest/config/__init__.py to use the warnings module

[gnikonorov]

Currently src/_pytest/config/__init__.py uses sys.stderr.write to output warnings. Building off of #7286, we should refactor this file to use the warnings module instead once the aforementioned PR is merged in

[gnikonorov]

I'll grab this after the mentioned PR is in master

[gnikonorov]

This should be done after #7305.
As it stands right now, converting the sys.stderr.write calls to warnings causes a bunch of tests to fail.

While we can get around it by placing the offending code in with.pytest.warns blocks. That seems like a hack. We should really add whitelists to all the tests that fail with this change as they properly deem keys inappropriate

[gnikonorov]

blocked by #7330 because it introduces a stderr warning. I have the change ready. I'll make a PR for it once #7330 is in