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

[ShadowJonathan]

Is your feature request related to a problem? Please describe.

Matrix now has a quarterly release cadence, starting with v1.1, it'd help if libraries and clients can start relying on these versions to introduce, work with, or otherwise stay compatible with versions and features per-version.

Describe the solution you'd like

A few action items;

• Document the matrix versions supported ("Compatible with matrix vX" badges/shields#7245)
  • Make supported matrix versions programmaticallu queryable
• Introduce compatibility probing (call /versions)
  • Bubble error when client cannot properly communicate to server.
    ("Cannot communicate with server, this library version 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

Describe alternatives you've considered

Additional context