Reset `sliding_sync_membership_snapshots` -> `forgotten` status when
membership changes (like rejoining a room).

Fix https://github.com/element-hq/synapse/issues/17781

### What was the problem before?

Previously, if someone used `/forget` on one of their rooms, it would
update `sliding_sync_membership_snapshots` as expected but when someone
rejoined the room (or had any membership change), the upsert didn't
overwrite and reset the `forgotten` status so it remained `forgotten`
and invisible down the Sliding Sync endpoint.