Upgrade Weblate to latest 4.x

In advance:

• Migrate the Translation Platform to use a containerized Weblate: #17870
• Check and eventually adapt our custom scripts to latest Weblate API
• Check whether there are any changes in relation to the current translation workflow
• Get puppet-tails!77 to a working merge-able state
• Check whether unicode-related errors occur in the upgraded Weblate. (can't reproduce myself, thus will rely on Reviewers testing)
• Check whether permissions enforcing works fine
• Coordinate testing of the upgraded platform
• Setup upgrade tooling in production VM
• Schedule downtime (current plan: Feb 15, 2022) and warn in advance

On the scheduled day:

• Shutdown the service: disable Puppet Agent and stop Apache and Weblate
• Disable cron jobs.
• Save a backup of Weblate
• Merge Weblate upgrade: puppet-tails!77
• Pull all intermediate container images needed for the upgrade.
• Change the database owner to weblate.
• Run Puppet Agent until it succeeds (2-3 times) and then shutdown the service again
• For each new version (4.0, ..., 4.9), do the upgrade dance: make upgrade VERSION=4.X (1h)
• Change the database owner back to postgres.
• Enable and run Puppet Agent
• Remove unnecessary container images.
• Make sure the website and all glue code works fine
• Report about permission changes in tails-weblate@.
• File issue to discuss and plan further upgrades (4.10 is already out) -- #17890