BulkIndexer change _routing -> routing to support ES 7

In ES 6.x onwards _routing is deprecated in the bulk API but in 7.x it
was removed completely. While this is sadly not clearly documented it's
referenced in a few places like
https://discuss.elastic.co/t/deprecated-field-routing-used-expected-routing-instead/115951
and we also learnt this when fixing the Go indexer at
https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/-/merge_requests/34

I have tested this version works for 6.5.1 , 6.8.1 and 7.5.1 .
Additionally since this is the same way that our
Go gitlab-elasticsearch-indexer is behaving this should be a safe
change.

See https://gitlab.com/gitlab-org/gitlab/-/issues/208717#note_300201720

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