Currently dbclient_rs_test and tenant_migration_recipient_service_test use the ScanningReplicaSetMonitor because it supports some mocking capabilities, and the StreamableReplicaSetMonitor does not.

Since we're removing production support for ScanningReplicaSetMonitor, we should also remove the need for it in unit tests.

Some potential ideas for implementation:
1. Add a lightweight MockReplicaSetMonitor that only does what's necessary
2. Add the ability to mock some behaviors of StreamableReplicaSetMonitor
3. Make the tests in question depend on higher level components rather than directly on the RSM and mock those instead
4. Keep the ScanningReplicaSetMonitor and only use it for tests (in which case we'd just close this ticket).

If the decided outcome involves the complete removal of the ScanningReplicaSetMonitor, this ticket should either remove all remaining references to the ScanningReplicaSetMonitor from the codebase, or if it makes more sense to do it separately, make a separate ticket to do that.