[Meta-issue] 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 (for if users are running versions on their own servers)
• 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

Android and iOS versions of this enhancement: element-hq/element-ios#5115, element-hq/element-android#4441

[dbkr]

This is quite broad and might be better off being several issues. That said, we already have a /versions call and the 'compatibility mode' you describe is effectively what we've already done for server that don't support spaces, for example.

[ShadowJonathan]

It might be better to expand that into matrix-js-sdk and such, with matrix versions in mind to merge compatibility for broader things.

I do agree it is quite broad, those action items should be individual issues, but i just wanted to throw this in here for now to branch it off later into seperate issues when the need arises. So this is a meta-issue, of sorts.

[turt2live]

Duplicate of #22346