Clean up `FillFileStore` background migrations with `BackgroundMigration.steal`
Description
We scheduled background migrations for filling ci_job_artifacts.file_store
, lfs_objects.file_store
and uploads.store
columns 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
Edited by Shinya Maeda