Closes#16054

This PR adds a new LoopService called DBVacuum that allows for the automatic clean up of Prefect resources that are older than a configurable retention period. This is to help keep the Prefect DB from overflowing with old flow runs/task runs/etc.

The DBVacuum deletes the following:

1. flow runs
2. associated logs
3. associated artifacts

Task runs are handled by the DELETE CASCADE rules setup on the flow run table.

The DBVacuum queries a limited amount of parent flow runs (those that do not have a parent_task_run_id) that are older than the retention and deletes them. It then deletes logs that no longer have any flow runs associated with them and finally performs the same deletion for artifacts.

The deletion of a parent flow run will delete child task runs and set their child flow runs parent_task_run_id to null, which will get picked up on the next round of the loop. This gives a guarantee that eventually all flow runs and their sub flow runs will be deleted (along with the associated logs and artifacts).

Checklist

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