Rotate offsite assets backups older than 30 days
Rotate offsite assets backups older than 30 days

The duplicity module doesn't do this by default so they will continue to
grow on the offsite backup machine. Rotate them after 30 days in line with
our offsite datastore backups:

- https://github.com/alphagov/govuk_offsitebackups-puppet/blob/8059d435975e352729597c6915f07f628df6e8fb/modules/base/manifests/rotate.pp

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