Always raise warnings for deprecated feature checks

[radarhere]

When I run the test suite without WebP installed, it fails.

Tests/test_features.py::test_check FAILED                                [  2%]

================================== FAILURES ===================================
_________________________________ test_check __________________________________

    def test_check() -> None:
        # Check the correctness of the convenience function
        for module in features.modules:
            assert features.check_module(module) == features.check(module)
        for codec in features.codecs:
            assert features.check_codec(codec) == features.check(codec)
        for feature in features.features:
            if "webp" in feature:
>               with pytest.warns(DeprecationWarning):
E               Failed: DID NOT WARN. No warnings of type (<class 'DeprecationWarning'>,) were emitted.
E                Emitted warnings: [].

The backstory of this is that #8213 deprecated features.check("transp_webp"), features.check("webp_mux") and features.check("webp_anim").

However, the deprecation warning is only raised if the underlying WebP module is installed, and the missing module doesn't raise an error earlier than the warning.

https://github.com/python-pillow/Pillow/blob/e93dcc15780d29af45fa64cf54ff6b11c2c27c46/src/PIL/features.py#L149-L159

This PR updates features.py so that the feature checks raise a deprecation warning when WebP is not installed as well.