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

alertmanager/kustomization.yaml

@@ -2,4 +2,4 @@ apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
 resources:
 - manifests/servicemonitor.yaml
-- vault/alertmanager-secrets.yaml
+- manifests/secret.yaml