Stop taking time-based APT snapshots of Buster

From the documentation:

After we don't need snapshots for a given Debian distribution anymore, e.g.
after we release Tails based on a new Debian, we need to stop including it in
new snapshots, and to remove all corresponding time-based snapshots and the
packages that are not referenced anymore.

Last time: #17165 (closed)

Steps to accomplish

• Upgrade po4a to 0.62 → tails#18667
• Release Tails 5.x, built with a Vagrant box that has the changes from step (1) applied, i.e. it does not need Buster snapshots anymore.
• Ditch Buster APT snapshots
  • Remove Buster from reprepro Time-based APT snapshots config -- puppet-tails!98
  • Remove files from apt.lizard