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

This issue has been migrated from [#14577](https://github.com/matrix-org/synapse/issues/14577).

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*Spawning from https://github.com/matrix-org/synapse/pull/14471#discussion_r1030803024 as mentioned by @squahtx,*

We can safely remove the following code that was introduced in https://github.com/matrix-org/synapse/pull/14471 when stabilizing the MSC3030 `/timestamp_to_event` endpoint:

https://github.com/matrix-org/synapse/blob/394d9171b16c481e842dd9cc29f477b0d7a4819a/synapse/federation/federation_client.py#L1694-L1696

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.