--warn-unused-config should not be a strict flag

[ilevkivskyi]

Fixes #21137

This flag has nothing to with type checking strictness, it is a convenience option for config maintenance.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

psycopg (https://github.com/psycopg/psycopg)
- pyproject.toml: note: unused section(s): module = ['polib', 'scrap.*']

mongo-python-driver (https://github.com/mongodb/mongo-python-driver)
- pyproject.toml: note: unused section(s): module = ['test.*']

sphinx (https://github.com/sphinx-doc/sphinx)
- pyproject.toml: note: unused section(s): module = ['pyximport', 'tests.test_config.test_config', 'tests.test_directives.test_directive_other', 'tests.test_domains.test_domain_c', 'tests.test_domains.test_domain_cpp', 'tests.test_domains.test_domain_js', 'tests.test_domains.test_domain_py', 'tests.test_domains.test_domain_py_fields', 'tests.test_domains.test_domain_py_pyfunction', 'tests.test_domains.test_domain_py_pyobject', 'tests.test_domains.test_domain_std', 'tests.test_environment.test_environment_toctree', 'tests.test_ext_autodoc.test_ext_autodoc', 'tests.test_ext_autodoc.test_ext_autodoc_mock', 'tests.test_ext_autosummary.test_ext_autosummary', 'tests.test_ext_intersphinx.test_ext_intersphinx', 'tests.test_ext_napoleon.test_ext_napoleon_docstring', 'tests.test_extensions.test_ext_apidoc', 'tests.test_extensions.test_ext_inheritance_diagram', 'tests.test_search', 'tests.test_util.test_util_inspect', 'tests.test_util.test_util_nodes', 'tests.test_util.test_util_typing', 'tests.test_util.typing_test_data']

svcs (https://github.com/hynek/svcs)
- pyproject.toml: note: unused section(s): module = ['conftest', 'health_check_pyramid', 'pytest.*']

pylint (https://github.com/pycqa/pylint)
- pyproject.toml: note: unused section(s): module = ['colorama', 'contributors_txt', 'coverage', 'git.*', 'mccabe', 'pytest_benchmark.*', 'sphinx.*']

dulwich (https://github.com/dulwich/dulwich)
- pyproject.toml: note: unused section(s): module = ['atheris', 'fastimport.*']

black (https://github.com/psf/black)
- pyproject.toml: note: unused section(s): module = ['tests.data.*']

packaging (https://github.com/pypa/packaging)
- pyproject.toml: note: unused section(s): module = ['pkg_resources', 'pretend', 'progress.*']

artigraph (https://github.com/artigraph/artigraph)
- pyproject.toml: note: unused section(s): module = ['sh']

bokeh (https://github.com/bokeh/bokeh)
- pyproject.toml: note: unused section(s): module = ['tests.*', 'tests.codebase.*', 'tests.support.defaults', 'tests.test_bokehjs', 'tests.test_cross', 'tests.test_defaults', 'tests.test_examples', 'tests.unit.bokeh.document.test_callbacks__document', 'tests.unit.bokeh.embed.test_standalone', 'tests.unit.bokeh.test_layouts__typing', 'tests.unit.bokeh.test_typing']

isort (https://github.com/pycqa/isort)
- pyproject.toml: note: unused section(s): module = ['importlib_metadata.*', 'tests.*']

nox (https://github.com/wntrblm/nox)
- pyproject.toml: note: unused section(s): module = ['tox.*']

boostedblob (https://github.com/hauntsaninja/boostedblob)
- pyproject.toml: note: unused section(s): module = ['tests.*', 'xmltodict.*']