Remove `failover_on_unknown_endpoint` from `/timestamp_to_event` after 2023-11-16

[MadLittleMods]

Spawning from #14471 (comment) as mentioned by @squahtx,

We can safely remove the following code that was introduced in #14471 when stabilizing the MSC3030 /timestamp_to_event endpoint:

synapse/synapse/federation/federation_client.py

Lines 1694 to 1696 in 394d917

	# Since this endpoint is new, we should try other servers before giving up.
	# We can safely remove this in a year (remove after 2023-11-16).
	failover_on_unknown_endpoint=True,

After a year, the endpoint will no longer be new and there will be many Synapse versions that include the stabilized endpoint.

It's possible that we leave this check in place though. Maybe we want to handle the case where another homeserver implementation doesn't have it available.