Quarantine a flaky test

The `` RuboCop::BatchedBackgroundMigrationsDictionary#finalize_after returns the finalize_after timestamp of the bbm with given version ``
test has either ~"flakiness::1" or ~"flakiness::2" 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](https://handbook.gitlab.com/handbook/engineering/infrastructure/engineering-productivity/flaky-tests-management-and-processes/#flaky-tests-management-process)
to help us increase `master` stability.

Please let us know your feedback in the
[Engineering Productivity issue tracker](https://gitlab.com/gitlab-org/quality/engineering-productivity/team/-/issues).

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/456914.

This change was generated by
[gitlab-housekeeper](https://gitlab.com/gitlab-org/gitlab/-/tree/master/gems/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.

Changelog: other

spec/rubocop/batched_background_migrations_dictionary_spec.rb

@@ -57,7 +57,8 @@
   end
 
   describe '#finalize_after' do
-    it 'returns the finalize_after timestamp of the bbm with given version' do
+    it 'returns the finalize_after timestamp of the bbm with given version',
+      quarantine: 'https://gitlab.com/gitlab-org/gitlab/-/issues/456914' do
       expect(batched_background_migration.finalize_after).to eq(finalize_after)
     end