Stabilise extended profiles (now part of Matrix v1.16)

[Half-Shot]

Requires #5014
Part of the spec since v1.16

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[t3chguy]

js-sdk claims it doesn't support v1.16 so that would probably need fixing first https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/version-support.ts#L26

[Half-Shot]

js-sdk claims it doesn't support v1.16 so that would probably need fixing first https://github.com/matrix-org/matrix-js-sdk/blob/develop/src/version-support.ts#L26

Oh god :(

[richvdh]

js-sdk claims it doesn't support v1.16 so that would probably need fixing first develop/src/version-support.ts#L26

not sure that's correct? wouldn't it be the other way around: we must support the stable names before declaring support for 1.16?

[richvdh]

cc @Half-Shot

[Half-Shot]

I mean it's up to the policy of the team. I'd have thought we can opportunistically support bits of v1.16 without having to declare support for it or it's predecessors? What's the problem we foresee with doing that?