chore: Adjust discovery ListenerClass

When running the getting started script against a local kind cluster,
stackablectl will automatically choose the stable-nodes preset, which
works out of the box with both external-stable and external-unstable.

However, when using helm, the default preset (ephemeral-nodes) is used
and causes OpenSearch cluster pods to not start up, because kind doesn't
support LoadBalancers. This is the reason why we chose to do the less
invasive change, and change the ListenerClass to external-unstable.

Author: Techassi

docs/modules/opensearch/examples/getting_started/opensearch.yaml

@@ -46,7 +46,7 @@ spec:
                 key: roles_mapping.yml
   nodes:
     roleConfig:
-      discoveryServiceListenerClass: external-stable
+      discoveryServiceListenerClass: external-unstable
     roleGroups:
       default:
         replicas: 3