Merge branch 'fix-bulk-indexer-es7-compatibility' into 'master'

BulkIndexer change _routing -> routing to support ES 7

See merge request gitlab-org/gitlab!26639

ee/changelogs/unreleased/fix-bulk-indexer-es7-compatibility.yml

+---
+title: BulkIndexer change _routing -> routing to support ES 7
+merge_request: 26639
+author:
+type: fixed

ee/lib/gitlab/elastic/bulk_indexer.rb

@@ -76,7 +76,7 @@ module Gitlab
           _id: ref.es_id
         }
 
-        op[:_routing] = ref.es_parent if ref.es_parent # blank for projects
+        op[:routing] = ref.es_parent if ref.es_parent # blank for projects
 
         op
       end