feat(vmuser): use secret instead of hardcoded value for vmuser authentication

otc/observability.t09.de/stacks/observability-client/vm-client-stack/manifests/simple-user-secret.yaml

@@ -1,9 +0,0 @@
-apiVersion: v1
-kind: Secret
-metadata:
-  name: simple-user-secret
-  namespace: observability
-type: Opaque
-stringData:
-  username: simple-user
-  password: simple-password

otc/observability.t09.de/stacks/observability/victoria-k8s-stack/manifests/vmauth.yaml

@@ -5,7 +5,9 @@ metadata:
   namespace: observability
 spec:
   username: simple-user
-  password: simple-password
+  passwordRef:
+    key: password
+    name: simple-user-secret
   targetRefs:
     - static:
         url: http://vmsingle-o12y:8429