REFACTOR(repo): move vault/ to manifests/

- Move ExternalSecret files from vault/ to manifests/secret.yaml
- Update kustomization.yaml references
- Remove vault/ folders

Apps: alertmanager, grafana, prometheus

prometheus/manifests/secret.yaml

@@ -0,0 +1,18 @@
+apiVersion: external-secrets.io/v1
+kind: ExternalSecret
+metadata:
+  name: postgresql-password
+  namespace: prometheus
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    kind: ClusterSecretStore
+    name: vault-backend
+  target:
+    name: postgresql-password
+    creationPolicy: Owner
+  data:
+    - secretKey: password
+      remoteRef:
+        key: databases/postgresql
+        property: PASSWORD