Stop sending incorrect knock_state_events. (#16403)

Synapse was incorrectly implemented with a knock_state_events
property on some APIs (instead of knock_room_state). This was
correct in Synapse 1.70.0, but *both* fields were sent to also be
compatible with Synapse versions expecting the wrong field.

Enough time has passed that only the correct field needs to be
included/handled.