Declare support for Matrix v1.9

[clokep]

See the changelog (note that I stripped out spec clarifications).

This also requires us to finish support for Matrix v1.8: see #16176.

Client-Server API

Backwards Compatible Changes

• Add the m.rule.suppress_edits default push rule, as per MSC3958.
  • Stabilize support for MSC3958 (suppress notifications from edits) #16113

[clokep]

So this is already done, pending #16176.