Upgrade our ISO builder Vagrant basebox to Stretch

Technically this is not required for Tails 3.0: especially if Jessie LTS is a thing, we can stick to Jessie for a while. But let’s keep this in mind and use Redmine relationships to indicate what this ticket is blocking, so that we can better prioritize it.

Feature Branch: feature/11738-stretch-vagrant-basebox

Related issues

Blocks #11912 (closed)
Blocked by #13530 (closed)

Original created by @intrigeri on 11738 (Redmine)

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