Epic: Support Gitter sunsetting

[Johennes]

Implement MSC3946 in matrix-js-sdk:

### Tasks
- [x] Tests for getVisibleRooms https://github.com/matrix-org/matrix-js-sdk/pull/3038
- [x] Factor out a function to find a room predecessor https://github.com/matrix-org/matrix-js-sdk/pull/3039
- [x] Tests for getRoomUpgradeHistory https://github.com/matrix-org/matrix-js-sdk/pull/3088
- [x] Fix the bug in getRoomUpgradeHistory (tests are cool) https://github.com/matrix-org/matrix-js-sdk/pull/3089
- [x] Refactor to use the new function for finding predecessors https://github.com/matrix-org/matrix-js-sdk/pull/3090
- [x] Adapt getVisibleRooms and getRoomUpgradeHistory to allow using predecessor events when enabled https://github.com/matrix-org/matrix-js-sdk/pull/3042
- [x] Add to the API the ability to get the tombstone event ID as well as the room predecessor ID (rename it findPredecessor) https://github.com/matrix-org/matrix-js-sdk/pull/3095

Implement MSC3946 in matrix-react-

### Tasks
- [x] vector-im/element-web#24322
- [x] ⭐ vector-im/element-web#24323
- [x] vector-im/element-web#24324
- [x] ⭐ vector-im/element-web#24325
- [x] vector-im/element-web#24326
- [x] vector-im/element-web#24327
- [x] vector-im/element-web#24328

Other

### Tasks
- [x] https://github.com/vector-im/element-web/issues/24417
- [ ] https://github.com/vector-im/element-web/issues/24453 
- [x] https://github.com/vector-im/element-web/issues/24611
- [x] https://github.com/vector-im/element-web/issues/24480

De-scoped for now:

• When viewing a room unauthenticated as a guest, it doesn't have any feature toggles enabled (like threads) #24513
• Filter out flood of member & hidden event spam when we detect the scrollback is full of it element-meta#1339
• To support accessing Gitter upload content in <img src="files.gitter.im/...">, replace external images with alt text instead of completely stripping them out #4044

Internal references

myhours: https://app.myhours.com/#/projects/1994529/overview

[Johennes]

Descoping #24513 as it's fairly low impact.

[Johennes]

Also descoping element-hq/element-meta#1339 since there is no clear general solution available right now and we have the general performance improvement of #24480 in scope.