Temporarily disable add-on purchase cleanup worker

What does this MR do and why?

We discovered that we rely on expired add-on purchases to not be cleaned up. As a temporary solution, we disable the cleanup worker's execution with this merge request to avoid an increasing number of namespaces that would receive an error on trial registration because they already had a trial provisioned in the past.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Go to http://localhost:3000/admin/background_jobs
2. Go to Cron
3. Check that gitlab_subscriptions_add_on_purchases_cleanup_worker is gone. The changes aren't reflected here. Does anyone have any ideas on that? 🤔

Resolves #478826