Rename fantom_internal_flags pragma to fantom_react_fb_flags (#49728)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49728

See title.

Resolving https://www.internalfb.com/diff/D70242739?dst_version_fbid=1295747645054627&transaction_fbid=1285615715828636

Changelog: [Internal]

Reviewed By: rubennorte

Differential Revision: D70328653

fbshipit-source-id: c3e7377760f53bc37d1a6f3e25a0053e12d4f66e