Update helm stable repo (#6867)

As https://helm.sh/blog/new-location-stable-incubator-charts/
helm stable repo is changed to https://charts.helm.sh/stable
In addition, if using helm v3.4.0+ the old stable repo installation
is failed.
So this updates the stable repo to avoid such error.

roles/kubernetes-apps/helm/defaults/main.yml

@@ -32,7 +32,7 @@ helm_cert_group: root
 helm_cert_owner: root
 
 # Set URL for stable repository
-# helm_stable_repo_url: "https://kubernetes-charts.storage.googleapis.com"
+# helm_stable_repo_url: "https://charts.helm.sh/stable"
 
 # Namespace for the Tiller Deployment.
 tiller_namespace: kube-system