upgrade to kubernetes 1.16 which should still work, add clarifying comment

ansible/roles/rke_configuration/files/cluster-defaults.yml

@@ -26,7 +26,9 @@ ingress:
   provider: none
 # If `kubernetes_version` is not set, the default kubernetes version for the
 # specified rke version in `ansible/group_vars/all/oas.yml` is used.
-kubernetes_version: 'v1.15.11-rancher1-3'
+# Currently pinned to newest 1.16 because of the problem described in this
+# comment: https://github.com/rancher/rancher/issues/14836#issuecomment-612590209
+kubernetes_version: 'v1.16.9-rancher1-1'
 monitoring:
   options: {}
   provider: ''