Set standby_leader_label_value for Patroni v4 Compatibility

Sets Patroni's 'standby_leader_label_value' setting to 'master' to make
standby clusters compatible with Patroni v4.

Issue: PGO-2293

Author: andrewlecuyer

internal/patroni/config.go

@@ -58,7 +58,9 @@ func clusterYAML(
 			"use_endpoints": true,
 			// To support transitioning to Patroni v4, set the value to 'master'.
 			// In a future release, this can be removed in favor of the default.
-			"leader_label_value": naming.RolePatroniLeader,
+			// Do this for leaders in both primary and standby clusters.
+			"leader_label_value":         naming.RolePatroniLeader,
+			"standby_leader_label_value": naming.RolePatroniLeader,
 
 			// In addition to "scope_label" above, Patroni will add the following to
 			// every object it creates. It will also use these as filters when doing