Warn users of Python 3.14+ against `cls.__dict__.get("__annotations__", {})`

[dylwil3]

Summary

cf #17658 (review) and #17658 (comment)

[AlexWaygood]

It might actually be good to emit this warning regardless of the target-version that Ruff infers. Just because somebody has target-version = "py38" in their config file doesn't mean that they're uninterested in their code working on Python 3.14. And there are more bulletproof ways of accessing the class's annotations that will work on all Python versions (either inspect.get_annotations or the typing_extensions backport of annotationlib.get_annotations).