Only return state events that the AS passed in via `state_events_at_start` (MSC2716) (#10552)

* Only return state events that the AS passed in via state_events_at_start

As discovered by @Half-Shot in
https://github.com/matrix-org/matrix-doc/pull/2716#discussion_r684158448

Part of MSC2716

* Add changelog

* Fix changelog extension