I messed this up a bit in #6805, but fortunately we weren't actually doing
anything with the room_version so it didn't matter that it was a str not a RoomVersion.

We were in fact only deleting stale marker when we got an incremental
update, rather than when we did a full resync.

Pass room_version into add_hashes_and_signatures

Make `get_room_version` return a RoomVersion object

So that we can start factoring out some of this boilerplatey boilerplate.

... to make way for a forthcoming get_room_version which returns a RoomVersion
object.