Clean stale background migration jobs

Stan Hurequested to merge
sh-cleaup-prom-background-migrations into master

In !24135 (merged), we reverted the problematic ActivatePrometheusServicesForSharedClusterApplications background migration, but the migration only dropped jobs that were in the background_migration Sidekiq queue. There may be more jobs in the Sidekiq scheduled or retry sets, so we need to clear those too.

Closes https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/9160

Edited by Yorick Peterse

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