Fixes#19347
This deprecates MSC2697 which has been closed since May 2024. As per
#19347 this seems to be a thing we can just rip out. The crypto team
have moved onto MSC3814 and are suggesting that developers who rely on
MSC2697 should use MSC3814 instead.
MSC2697 implementation originally introduced by https://github.com/matrix-org/synapse/pull/8380