fix: Use admin.existingSecret for Grafana creds from Doppler

Revert to idiomatic Grafana chart approach. ExternalSecret creates the
secret with admin-user/admin-password keys before Grafana's first start
on fresh cluster creation.

infrastructure/addons/observability/helmrelease-kube-prometheus-stack.yaml

@@ -21,18 +21,11 @@ spec:
   upgrade:
     remediation:
       retries: 3
-  valuesFrom:
-    - kind: Secret
-      name: grafana-admin-credentials
-      valuesKey: admin-user
-      targetPath: grafana.admin.user
-    - kind: Secret
-      name: grafana-admin-credentials
-      valuesKey: admin-password
-      targetPath: grafana.admin.password
   values:
     grafana:
       enabled: true
+      admin:
+        existingSecret: grafana-admin-credentials
       grafana.ini:
         server:
           root_url: http://grafana.silverside-gopher.ts.net/