Add Dokku postgres backup file pruner

[philbudne]

I've written most of a program that uses the boto3 S3 API (the lingua franca of the cloud storage world?) to inventory a bucket containing postgres backups, and determine which should be kept.

The only part remaining to be written is deleting the objects.
For S3, if bucket versioning is enabled, we may have to provide the VersionId for each object?

The API includes a bulk delete call:
https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/s3/client/delete_objects.html

[philbudne]

Code checked in to https://github.com/mediacloud/system-dev-ops/tree/main/postgres/prune-backups
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