fix(observability): 🐛 disable false-positive control-plane alerts and fix empty cluster_environment label

Hub defaultRules groups kubernetesSystemControllerManager, kubeScheduler, and
kubernetesSystemScheduler used wrong key 'enabled: false' — chart expects 'create: false'.
This caused KubeControllerManagerDown/KubeSchedulerDown to fire as false positives
because OTC CCE managed k8s does not expose control plane for scraping.

Dev local vmagent had empty externalLabels, so backup-alert rules evaluated by local
vmalert had no cluster_environment label on kube_job_status_failed metrics. Added
cluster_environment=dev to match what the vm-client-stack vmagent adds for hub shipping.

otc/dev.t09.de/stacks/observability/victoria-k8s-stack/values.yaml

@@ -708,7 +708,8 @@ vmagent:
     port: "8429"
     selectAllByDefault: true
     scrapeInterval: 20s
-    externalLabels: {}
+    externalLabels:
+      cluster_environment: "dev"
       # For multi-cluster setups it is useful to use "cluster" label to identify the metrics source.
       # For example:
       # cluster: cluster-name

otc/observability.buildth.ing/stacks/observability/victoria-k8s-stack/values.yaml

@@ -201,13 +201,13 @@ defaultRules:
       enabled: true
       rules: {}
     kubernetesSystemControllerManager:
-      enabled: false
+      create: false
       rules: {}
     kubeScheduler:
-      enabled: false
+      create: false
       rules: {}
     kubernetesSystemScheduler:
-      enabled: false
+      create: false
       rules: {}
     kubeStateMetrics:
       enabled: true