Deprecate #gitter_xxx=2Fxxx:matrix.org rooms on Matrix (=2F)

Spawned from #2674 (closed)

The new bridge is adding new #*:gitter.im rooms under it's own homeserver. We don't plan to support the old matrix.org =2F Gitter rooms, ex. #gitter_community=2Froom:matrix.org to the new bridge.

Maybe we can use Matrix tombstone event to upgrade people to the new rooms for these but that has it's own problems