fix nextcloud db-username + k8s-dashboard chart repo

- nextcloud: add db-username to ESO secret template and usernameKey
  to chart values (required by newer chart version)
- k8s-dashboard: update chart repo URL to kubernetes-retired.github.io
  (old kubernetes.github.io/dashboard returns 404)

stacks/nextcloud/main.tf

@@ -82,6 +82,7 @@ resource "kubernetes_manifest" "db_external_secret" {
         template = {
           data = {
             DB_PASSWORD = "{{ .password }}"
+            db-username = "nextcloud"
           }
         }
       }