Fix `warning_filter` warning

[msabramo]

Prevents this warning from MkDocs 1.6.0:

$ pdm run mkdocs build
INFO    -  DeprecationWarning: warning_filter doesn't do anything since MkDocs 1.2 and will be removed soon. All messages on the
           `mkdocs` logger get counted automatically.
             File
           ".../.venv/lib/python3.10/site-packages/mkdocs_ezlinks_plugin/plugin.py",
           line 5, in <module>
               from mkdocs.utils import warning_filter
             File ".../.venv/lib/python3.10/site-packages/mkdocs/utils/__init__.py",
           line 403, in __getattr__
               warnings.warn(