Temporarily change pytest.PytestRemovedIn9Warning error to warning

Temporarily change pytest.PytestRemovedIn9Warning error to warning

We need this to be able to use pytest 9.0 from Debian "sid".

Additionally, this error needs to be addressed before pytest 9.1 is
released, as the filter will no longer work. Also, all pytests in CI
images need to be upgraded to version 7, where the new API is supported.

We can't have the "filterwarnings" section in pytest.ini directly as
only pytest 8.0 knows about it.

(cherry picked from commit 0c5d9c00ef595f81afe8c0c7c2e9ee2128e13284)