Fix `get_last_event_in_room_before_stream_ordering(...)` finding the wrong last event (#17295)

PR where this was introduced: https://github.com/matrix-org/synapse/pull/14817

### What does this affect?

`get_last_event_in_room_before_stream_ordering(...)` is used in Sync v2 in a lot of different state calculations.

`get_last_event_in_room_before_stream_ordering(...)`  is also used in `/rooms/{roomId}/members`