Remove the purge CronJob and create a MedusaPurgeSchedule API

[adejanovski]

The CronJob we recently introduced is both buggy and full of unconfigurable settings which create more problems than it fixes.
We should remove it altogether and create a new MedusaPurgeSchedule API which will be copied off from the MedusaBackupSchedule API to control cyclic execution of purges.

While we're removing the creation of new purge CronJobs, we also need to clean up existing CronJobs to block further executions on existing clusters.

Definition of Done

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1. A MedusaPurgeSchedule API is created to handle cyclic executions of the purge MedusaTask
2. The purge CronJob is removed and a clean up is added for existing objects