Update high-traffic table links on database doc

Updates not null constraint documentation to reference the new
high-traffic table definition

doc/development/database/not_null_constraints.md

@@ -207,7 +207,7 @@ end
 
 ## `NOT NULL` constraints on large tables
 
-If you have to clean up a text column for a really [large table](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/migration_helpers.rb#L12)
+If you have to clean up a text column for a [high-traffic table](../migration_style_guide.md#high-traffic-tables)
 (for example, the `artifacts` in `ci_builds`), your background migration will go on for a while and
 it will need an additional [background migration cleaning up](../background_migrations.md#cleaning-up)
 in the release after adding the data migration.