REFACTOR(longhorn): change Longhorn namespace

- Change from longhorn-system to longhorn
- Standardize namespace naming

longhorn/argocd.yaml

@@ -23,7 +23,7 @@ spec:
     kustomize: {}
   destination:
     server: https://kubernetes.default.svc
-    namespace: longhorn-system
+    namespace: longhorn
   syncPolicy:
     automated:
       prune: true