rubocop: Add namespace_settings high traffic table

Add `namespace_settings` to the list of high traffic tables. It's a
table with about an equal amount of rows as `namespaces`, and that table
is also considered large, on both number of rows and total size.

At the time of writing:

| table              |   # rows |
|--------------------+----------|
| namespaces         | 10416978 |
| namespace_settings | 10319252 |

rubocop/rubocop-migrations.yml

@@ -19,6 +19,7 @@ Migration/UpdateLargeTable:
     - :merge_request_diffs
     - :merge_request_metrics
     - :merge_requests
+    - :namespace_settings
     - :namespaces
     - :note_diff_files
     - :notes