Gitter sunsetting: Adapt implementation to make use of via_servers property in m.predecessor event

[andybalaam]

The latest version of MSC3946: Dynamic room predecessor includes a via_servers property that helps us find a predecessor room if the user has never opened it before.

Adapt the room predecessor code to use store and this property where it helps.

[andybalaam]

This is just a re-articulation of #24453. Closing.