Upgrade Weblate to latest 3.x

Originally created by @zen on #17345 (Redmine)

Weblate releases more often than we've been able to upgrade. Upgrading Weblate is a time consuming task because:

• Some minor versions can’t be skipped.
• There's (still) some manual work involved in managing dependencies.
• Problems may arise during Django migrations.
• Our own glue code may be affected by changes in Weblate Python API.

Despite that, it’s better do it earlier (or frequently) than accumulate.

Ref: https://docs.weblate.org/en/latest/admin/upgrade.html#

Related issues

• Related to #17445 (closed)
• Blocks #17476 (closed)

Weblate Versions

This is the list of versions (as of today) that we need to migrate to, in order:

• 3.6.1
• 3.7.1
• 3.8
• 3.9.1
• 3.10.3
• 3.11.3
• 4.x (#17782 (closed))