When featureReplicaSetEndpoint is enabled (i.e. starting in 8.3), $backupCursor and $backupCursorExtend will only be supported against 'local' database instead of against any databases (SERVER-87299). File-based initial sync and shard merge are the internal users of $backupCursor and $backupCursorExtend and have been running them against the 'admin' database. So they need to be changed to run $backupCursor and $backupCursorExtend against the 'local' database instead.