Stabilise MSC4222: support the stable use_state_after identifiers on /sync

[barodeur]

Part of #19414 (Support Matrix 1.16).

⚠️ Before merging, wait for the following PRs to be merged first:

• Fix leave event missing from state_after on incremental sync (#18793) #20081
• Fix state_after for left rooms including post-leave lazy-loaded memberships #20169
• Fix state events missing from MSC4222 state_after when the since token falls inside a persist batch #20171

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Since Matrix 1.16, the spec defines the stable identifiers for MSC4222: the use_state_after query parameter on GET /_matrix/client/v3/sync, and the state_after room response field:

use_state_after — boolean — Controls whether to receive state changes between the previous sync and the start of the timeline, or between the previous sync and the end of the timeline. If this is set to true, servers MUST respond with the state between the previous sync and the end of the timeline in state_after and MUST omit state. […]

state_after — […] Required if use_state_after was set to true in the request, even if it is empty. Added in v1.16

— Matrix Spec

Synapse so far only implemented the unstable identifiers (org.matrix.msc4222.use_state_after / org.matrix.msc4222.state_after), gated behind the msc4222_enabled experimental flag and the per-user experimental features admin API.

This PR:

• accepts the stable use_state_after query parameter, unconditionally, and responds with the stable state_after field. The unstable query parameter is kept for a transition period; clients opting in with it get the unstable response field name back, so unstable-era clients see an unchanged wire format. If both parameters are set, the stable one wins.
• removes the msc4222_enabled experimental flag and the msc4222 per-user feature from the experimental features admin API, since the behaviour is no longer gated.

Pull Request Checklist

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