Rubocop: namespace_settings is high traffic table

What does this MR do?

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

Does this MR meet the acceptance criteria?

Conformity

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