Refactor `create_new_client_event` to use a new parameter, `state_event_ids`, which accurately describes the usage with MSC2716 instead of abusing `auth_event_ids` (#12083)

Spawned from https://github.com/matrix-org/synapse/pull/10975#discussion_r813183430

Part of [MSC2716](https://github.com/matrix-org/matrix-spec-proposals/pull/2716)