Clean up `ArchiveLegacyTraces` background migrations with `BackgroundMigration.steal`
Description
We scheduled background migrations for archiving
legacy traces in %11.0. We need to clean up the remaining background migration queues in %11.1.
Referece: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/background_migrations.md
Importance
We have to finish this background migrations ASAP. Because we can't modify ci_job_artifacts
table until it's done. We've already been suffered by this problem with JUnit feature, legacy artifact migrations.
Or make this migrations no-op would be the fastest solution.
Edited by Shinya Maeda