Threads: Switch to stable prefixes

### Description

We'd like to drop support for the unstable identifiers related to threads in Synapse. These span MSC3440, MSC3715, MSC3771, MSC3773, and MSC3856 which are all part of Matrix 1.4. To make this switch clients need to do the following:

* [ ] Switch to using `v1.4` (or `v1.5`) in the `/versions` response instead of:
* `org.matrix.msc3440.stable`
* `org.matrix.msc3771`
* `org.matrix.msc3773`
* [ ] Use `dir` instead of `org.matrix.msc3715.dir` when fetching from `/messages`.
* [ ] Use `unread_thread_notifications` instead of `org.matrix.msc3773.unread_thread_notifications` when:
* [ ] Generating room filters.
* [ ] Parsing sync responses.
* [ ] Stop using `related_by_senders` and `related_by_rel_types`, use the threads list API from MSC3856 instead.

Blocks matrix-org/synapse#14104.

(I'm assuming this is all handled in the SDK, but that may be incorrect!)