[#56716202] Enable unattended upgrades

Using new support in upstream apt module. The `mail_to` param will only be
set for production so as to prevent us getting spammed about Vagrant nodes
that we don't really care about. Done in:

gds/govuk_mirror-deployment@2fab191df0c96868e2c5397b181e891f155c65ba
Merge pull request #18 from alphagov/56716202-unattended_upgrades

[#56716202] Enable unattended upgrades