event properties in API responses and AS API duplicated as top-level field

This issue has been migrated from [#7925](https://github.com/matrix-org/synapse/issues/7925).

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When calling the `/messages` endpoint (also `/search`, `/context`, etc) each event in the `chunk` will have a number of top-level properties that duplicate those within `unsigned`:

* `age` (see also: #8429)
* `redacted_because`
* `replaces_state`
* `invite_room_state`
* `knock_room_state`.
* `prev_content` (though this one was specced until [MSC3442](https://github.com/matrix-org/matrix-doc/blob/main/proposals/3442-move-prev-content.md))
* `user_id` (duplicates `sender`)

These top-level properties are not a part of the spec.

(Edited by @richvdh to make the list of affected properties clear)