Related to https://github.com/matrix-org/synapse/pull/15491

This doesn't hurt to be done early on, while still on Synapse v1.83.0.

We'll be able to remove the `worker_replication_*` settings later,
when Synapse v1.84.0 gets released and starts making use of the new
`main` instance in the instance map instead of looking at the
`worker_replication_*` settings.