Warn on upload for deprecated metadata fields

[di]

What's the problem this feature will solve? In https://github.com/pypi/warehouse/issues/17389 we discovered that some build backends are still using deprecated metadata fields, and haven't started warning emails to stop using these fields.

Describe the solution you'd like We should warn users via email that these fields are deprecated when they are provided via upload.

Additional context https://packaging.python.org/en/latest/specifications/core-metadata/#deprecated-fields