Quarantine a flaky test

The RuboCop::BatchedBackgroundMigrationsDictionary#introduced_by_url returns the introduced_by_url of the bbm with given version test has either flakiness1 or flakiness2 label set, which means the number of reported failures is at or above 95 percentile, indicating unusually high failure count.

This MR quarantines the test. This is a discussion starting point to let the responsible group know about the flakiness so that they can take action:

• accept the merge request and schedule the associated issue to improve the test
• close the merge request in favor of another merge request to delete the test

Please follow the Flaky tests management process to help us increase master stability.

Please let us know your feedback in the Engineering Productivity issue tracker.

Related to #456912 (closed).

This change was generated by gitlab-housekeeper using the Keeps::QuarantineFlakyTests keep.

To provide feedback on your experience with gitlab-housekeeper please create an issue with the label GitLab Housekeeper and consider pinging the author of this keep.

spec/rubocop/batched_background_migrations_dictionary_spec.rb

@@ -46,7 +46,8 @@
   end
 
   describe '#introduced_by_url' do
-    it 'returns the introduced_by_url of the bbm with given version' do
+    it 'returns the introduced_by_url of the bbm with given version',
+      quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/456912' do
       expect(batched_background_migration.introduced_by_url).to eq(introduced_by_url)
     end