Upgrade to po4a 0.62

po4a 0.62 (Bullseye) generates different PO files than 0.55 (Buster), so we have to stick to the version from Buster (0.55) until we coordinate the upgrade of this package on RMs' systems, Vagrant build box, www.lizard, and translate.lizard. Goal: avoid making tons of strings fuzzy, back'n'forth, when updating PO files with a different version of po4a.

Affected systems, people, and files:

• Get an informal lock on the master branch
  • Ask developers and technical writers not to push to master
  • Disable the cronjob where weblate pushes to the master branch
• Prepare a branch that re-generates PO files on the master branch with the new po4a (!833 (merged))
• In lock-step
  • Update po4a on www.lizard: ikiwiki refreshes PO files (puppet-tails!91 (merged)), then immediately:
  • Merge the branch that updates PO files (!833 (merged))
• Wait until:
  • the CI pipeline passes on master
  • ikiwiki finishes refreshing the production website
  • the production website looks OK
• Vagrant build box
  • update vagrant/definitions/tails-builder/generate-tails-builder-box.sh on the stable branch (!834 (merged))
• translate.lizard: for the staging website
• every RM's system: we update PO files as part of the release process
  • update wiki/src/contribute/release_process.mdwn (!833 (merged))
  • notify release managers
• technical writers: they sometimes update PO files as part of their work
  • update wiki/src/contribute/build/website.mdwn (!833 (merged))
  • notify technical writers
• Re-enable Puppet agent on translate.lizard
• Ensure Weblate picks up the updated strings