[API] Generator: Use full path for Utils since used in X-Pack too

Author: picandocodigo

elasticsearch-api/utils/thor/generator/endpoint_specifics.rb

@@ -152,7 +152,7 @@ def msearch_template_body_helper
       def bulk_body_helper
         <<~SRC
           if body.is_a? Array
-            payload = Utils.__bulkify(body)
+            payload = Elasticsearch::API::Utils.__bulkify(body)
           else
             payload = body
           end