Gitter sunsetting: ensure hidden highlights code respects MSC3946

[Johennes]

Adapt the code inside RoomView to call findPredecessor instead of manually extracting room predecessor from the create event.

Pass through the MSC3946 feature flag to findPredecessor to ensure it has the correct behaviour.

[andybalaam]

Chatting to Eric this morning, #24325 seems more urgent.

Meanwhile, testing RoomView is proving very annoying, so putting this on hold and picking up 24325

[andybalaam]

I looked at this briefly and found testing RoomView tricky. Feel free to give me a ping if you'd like to talk it through.

[andybalaam]

This change only affects whether you see "hidden highlights": notifications from an old room. This does not seem important enough to block Gitter sunsetting.