backport of commit 4a7bee5a02 (#24331)

Co-authored-by: Mike Palmiotto <mike.palmiotto@hashicorp.com>

changelog/24325.txt

@@ -0,0 +1,4 @@
+```release-note:change
+identity (enterprise): POST requests to the `/identity/entity/merge` endpoint
+are now always forwarded from standbys to the active node.
+```

vault/identity_store_entities.go

@@ -231,7 +231,8 @@ func entityPaths(i *IdentityStore) []*framework.Path {
 			},
 			Operations: map[logical.Operation]framework.OperationHandler{
 				logical.UpdateOperation: &framework.PathOperation{
-					Callback: i.pathEntityMergeID(),
+					Callback:                  i.pathEntityMergeID(),
+					ForwardPerformanceStandby: true,
 				},
 			},