Add rncore stub headers for backwards-compat (#52083)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52083

These headers were removed in D55037569 but we may have some targets still depending on them. Add redirection headers with warnings to help users migrate without this being a breaking change.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D76810433

fbshipit-source-id: 43cddcc69eefbcff0c0140e165fb893bee493c79