Remove failover_on_unknown_endpoint from /timestamp_to_event after 2023-11-16

[matrixbot]

This issue has been migrated from #14577.

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Spawning from matrix-org/synapse#14471 (comment) as mentioned by @squahtx,

We can safely remove the following code that was introduced in matrix-org/synapse#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.