Remove/update transitional packages from install process

[legoktm]

Description

securedrop/install_files/ansible-base/roles/prepare-servers/tasks/main.yml

Line 6 in bbf1434

raw: apt-get update && apt-get install -y python3 apt-transport-https dnsutils ubuntu-release-upgrader-core mokutil

• apt-transport-https is obsolete now and can be removed.
• dnsutils has been renamed to bind9-dnsutils.

[zenmonkeykstop]

Would it be worth explicitly switching URLs in sources.list to HTTPS versions?

[legoktm]

Ubuntu still doesn't provide packages over HTTPS, see #3286 + https://bugs.launchpad.net/ubuntu/+bug/1464064. We would have to switch to a mirror that offered HTTPS, like kernel.org.