v12 upgrade disclaimer insincere

[bkil]

The following message is presented before a room upgrade to v12:

This action is irreversible! Options Version 12 Founders Special privileged users can be assigned during creation. These users have elevated control and can only be modified during a upgrade.

I think a tombstone should be technically reversible if you do it quick enough.

The set of additional_creators would indeed be baked into the create event. However, for normal use cases, one allowed to upgrade the room can still override this (see #2435). It is normal for an admin to elect backup admins to this level on demand to override this after each baked in creator (or their HS) is hit by the bus as we go.