scheduler: run celery backend cleanup when setup (have config option)

[yashlamba]

Celery runs a backend cleanup task by default, which has been removed in the scheduler: https://github.com/celery/celery/blob/21548ba81363d444878b67bdb8dddaf073e93eb8/celery/beat.py#L268-L274

We should ideally be running this task, it has been removed since the scheduler deployment with the existing scheduler is not final. This is to avoid overlapping tasks.