Add gitlab_subscriptions to high traffic tables

doc/development/migration_style_guide.md

@@ -369,6 +369,7 @@ migration involves one of the high-traffic tables:
 - `users`
 - `projects`
 - `namespaces`
+- `gitlab_subscriptions`
 - `issues`
 - `merge_requests`
 - `ci_pipelines`

rubocop/rubocop-migrations.yml

@@ -11,6 +11,7 @@ Migration/UpdateLargeTable:
     - :ci_stages
     - :deployments
     - :events
+    - :gitlab_subscriptions
     - :issues
     - :merge_request_diff_commits
     - :merge_request_diff_files