Since we don't actually delete the keys, just mark the versions
as deleted in the db rather than actually deleting them, then we
won't reuse versions.
Fixes https://github.com/vector-im/riot-web/issues/7448
Continues from uhoreg's branch
This just fixed the errcode on /room_keys/version if no backup and
updates the schema delta to be on the latest so it gets run
David Baker