Cop/IgnoredColumns: Flag use of `self.ignored_columns = [...]`

What does this MR do and why?

Previously, this cop only flagged self.ignored_columns += [...].

Because of this change the cop rule is put back in "grace period" to maintain master stability.

Stumbled upon this while working on #382533 (comment 1392132157).

🆕 Offenses

This rule change triggered the following 🆕 offenses which are excluded via .rubocop_todo/cop/ignored_columns.yml.

Offenses:

app/models/loose_foreign_keys/deleted_record.rb:9:8: C: Cop/IgnoredColumns: Use IgnoredColumns concern instead of setting self.ignored_columns.
  self.ignored_columns = %i[partition]
       ^^^^^^^^^^^^^^^
ee/lib/ee/gitlab/background_migration/create_vulnerability_links.rb:84:16: C: Cop/IgnoredColumns: Use IgnoredColumns concern instead of setting self.ignored_columns.
          self.ignored_columns = [:partition_number]
               ^^^^^^^^^^^^^^^
ee/lib/ee/gitlab/background_migration/migrate_vulnerabilities_feedback_to_vulnerabilities_state_transition.rb:63:16: C: Cop/IgnoredColumns: Use IgnoredColumns concern instead of setting self.ignored_columns.
          self.ignored_columns = [:partition_number]
               ^^^^^^^^^^^^^^^
spec/lib/gitlab/database/partitioning/sliding_list_strategy_spec.rb:198:16: C: Cop/IgnoredColumns: Use IgnoredColumns concern instead of setting self.ignored_columns.
          self.ignored_columns = [:partition]
               ^^^^^^^^^^^^^^^
spec/lib/gitlab/database/partitioning/sliding_list_strategy_spec.rb:217:14: C: Cop/IgnoredColumns: Use IgnoredColumns concern instead of setting self.ignored_columns.
        self.ignored_columns = %w[partition]
             ^^^^^^^^^^^^^^^

4 files inspected, 5 offenses detected

How to set up and validate locally

bundle exec rubocop --only Cop/IgnoredColumns

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.