pg backups filling up on bungei

similar to #41361 (closed) except now it's the /srv/backups/pg partition that's filling up...

1 year graph:

https://grafana.torproject.org/d/zbCoGRjnz/disk-usage?orgId=1&from=now-1y&to=now&var-class=All&var-instance=bungei.torproject.org

30 days:

https://grafana.torproject.org/d/zbCoGRjnz/disk-usage?orgId=1&from=now-30d&to=now&var-class=All&var-instance=bungei.torproject.org

change rate is -1TB per month according to grafana.

/cc @gk