If an old room version forgets it's been upgraded we should still ignore it if we are in a newer version of the room

[ara4n]

No description provided.

[tulir]

This sounds dangerous, since anyone can create a room with any predecessor they want. If someone then joined a room with a malicious predecessor without realizing it, they might have some rooms inexplicably disappear from their room list.

There might also be legitimate use cases for a predecessor that isn't an upgrade, e.g. forking a room into a more specific topic

[turt2live]

We could probably cache the tombstones for rooms more aggressively than other state, though it starts to be subject to #7843