Document supported matrix versions, and introduce compatibility modes for older ones

[ShadowJonathan]

Your use case

What would you like to do?

• Document the matrix versions supported ("Compatible with matrix vX" badges/shields#7245)
  • Document this within the client
• Introduce compatibility probing (call /versions)
  • Show user-facing error when server and client supported versions mismatch
    ("Cannot communicate with server, your client supports versions v1.1 and v1.2, but the server only supports v1.0")
• Introduce compatibility modes based off of the /versions response for older versions

Why would you like to do it?

Matrix now has a quarterly release cadence, it'd be nice to make element compatible with older matrix versions in a easily understandable way. (So that users may check compatibility for themselves)

Have you considered any alternatives?

No response

Additional context

No response