Mark background migrations as throttled

What does this MR do?

We have no particular requirements for the processing of this queue, as it's meant to be for long-running operations.

See gitlab-com/gl-infra/scalability#211 (closed).

I tested that this would still be picked up with our catchall node's configuration: https://ops.gitlab.net/gitlab-cookbooks/chef-repo/-/blob/master/roles/gprd-base-be-sidekiq-catchall.json

And it will:

$ bin/sidekiq-cluster --dryrun --experimental-queue-selector --negate "feature_category=global_search&urgency=throttled|resource_boundary=cpu&urgency=default,low|resource_boundary=memory|resource_boundary=cpu&urgency=high|resource_boundary!=cpu&urgency=high|name=chaos:chaos_cpu_spin,chaos:chaos_db_spin,chaos:chaos_kill,chaos:chaos_leak_mem,chaos:chaos_sleep,cronjob:geo_container_repository_sync_dispatch,cronjob:geo_file_download_dispatch,cronjob:geo_metrics_update,cronjob:geo_migrated_local_files_clean_up,cronjob:geo_prune_event_log,cronjob:geo_repository_sync,cronjob:geo_repository_verification_primary_batch,cronjob:geo_repository_verification_secondary_scheduler,cronjob:geo_repository_verification_secondary_shard,cronjob:geo_scheduler_per_shard_scheduler,cronjob:geo_scheduler_primary_per_shard_scheduler,cronjob:geo_scheduler_secondary_per_shard_scheduler,cronjob:geo_secondary_registry_consistency,default,geo:geo_batch_project_registry,geo:geo_batch_project_registry_scheduler,geo:geo_container_repository_sync,geo:geo_design_repository_shard_sync,geo:geo_design_repository_sync,geo:geo_event,geo:geo_file_download,geo:geo_file_registry_removal,geo:geo_file_removal,geo:geo_hashed_storage_attachments_migration,geo:geo_hashed_storage_migration,geo:geo_project_sync,geo:geo_rename_repository,geo:geo_repositories_clean_up,geo:geo_repository_cleanup,geo:geo_repository_destroy,geo:geo_repository_shard_sync,geo:geo_repository_verification_primary_shard,geo:geo_repository_verification_primary_single,geo:geo_repository_verification_secondary_single,geo:geo_scheduler_primary_scheduler,geo:geo_scheduler_scheduler,geo:geo_scheduler_secondary_scheduler,geo:geo_secondary_repository_backfill,hashed_storage:hashed_storage_migrator,hashed_storage:hashed_storage_project_migrate,hashed_storage:hashed_storage_project_rollback,hashed_storage:hashed_storage_rollbacker,ldap_group_sync,project_export" | grep -o background_migration
background_migration
background_migration