[Docs] Document license_file deprecation

Summary

setuptools emits a warning about setup.cfg usage of license_file, encouraging a switch to license_files, and links to https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

        ********************************************************************************
        The license_file parameter is deprecated, use license_files instead.

        This deprecation is overdue, please update your project and remove deprecated
        calls to avoid build errors in the future.

        See https://setuptools.pypa.io/en/latest/userguide/declarative_config.html for details.
        ********************************************************************************

But that link doesn't include any details on the deprecation.

OS / Environment

setuptools-68.2.2

Additional Information

Would be nice to see when the deprecation happened, a schedule for removal, etc. Maybe in the table on the linked docs page.

Code of Conduct

• I agree to follow the PSF Code of Conduct

Start with the linked declarative configuration documentation and locate the section or table covering license_file and license_files. Research the deprecation timing and removal schedule from the project history, then add those details to the relevant documentation page and verify that the warning links to the updated explanation.