Update room name of 1:1 chats other person has left

This has been changed in Riot in https://github.com/vector-im/riot-web/issues/6083 (although neither the old nor the new behaviour match the spec).

Once we've figured out what the actual correct answer is here, we should update the spec accordingly.

Start by reading Riot issue 6083 and determining the correct behavior against the Matrix specification. Once the behavior is settled, update the relevant specification text; done means the specification reflects the agreed behavior.

Written by the indexing model from the issue text.