Premake 4 is not used anymore, so we should drop support and rejoice ? I propose we do this after the release. That way, if something unexpected happens to the packagers, they can still build the game with --premake4 and release A23 without a headache.
Note: I didn't include the removal of build/premake/premake4 because that clutters the diff. This will be done upon committing. You can remove it manually for testing, but since the lua files for p4 are deleted, if this diff breaks something, it will break even without deleting the folder.