Mark data streams stats API as internal-only (again)

This is a redo of elastic#108745 which was reverted. Now that elastic#112303 has been merged, there is an alternative to retrieve the `maximum_timestamp`.

modules/data-streams/src/main/java/org/elasticsearch/datastreams/rest/RestDataStreamsStatsAction.java

@@ -22,7 +22,7 @@
 
 import static org.elasticsearch.rest.RestRequest.Method.GET;
 
-@ServerlessScope(Scope.PUBLIC)
+@ServerlessScope(Scope.INTERNAL)
 public class RestDataStreamsStatsAction extends BaseRestHandler {
     @Override
     public String getName() {