TPA-RFC-63: consider next steps for the backup server (bungei)

bungei filled up this week (#41361 (closed)) and while we mitigated this by allocating more space to the logical volume, there is now very little space in the volume group to dodge similar bullets in the future ("only" 2.6TB):

root@bungei:~# vgs
  VG      #PV #LV #SN Attr   VSize  VFree
  vg_bulk   1   2   0 wz--n- 72.60t 2.60t

tasks:

• review past tickets about bungei filling up
• review last years disk stats to see if there's another anomaly
• evaluate costs of a server replacement (see #41536 (closed))
• adopt budget for new storage server: https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-63-storage-server-budget
• build new storage server (spin out in a different issue?) (#41557 (closed))
• evaluate possible software replacements (see #40950 (closed) for PostgreSQL, and #41805 (closed) for bacula)