Switch to stable thread identifiers

[clokep]

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.