Validate-apt-version task may not be needed on Focal

[rocodes]

Description

validate-apt-version, which is named as a Xenial task, runs on Focal installs:
https://github.com/freedomofpress/securedrop/blob/012d30e8baeb8ffa7c10a56f4b43c464abf87cf7/install_files/ansible-base/roles/install-fpf-repo/tasks/validate_apt_version.yml

According to https://nvd.nist.gov/vuln/detail/CVE-2019-3462, the versions of apt that ship with Ubuntu 20.04 are not affected, so it looks like we can stop running this check.

Steps to Reproduce

./securedrop-admin install on Focal 20.04 servers

Comments

check when: ansible_distribution_release == 'focal' or remove entirely as we phase out xenial code

[conorsch]

Thanks, @rocodes. You're right, we can safely remove that code now. Ideally we'd be able to use HTTPS across the board for all apt repos, to be a bit more defensive against similar vulnerabilities in the future, but many of the Canonical repos are still HTTP-only. The FPF apt repo is already HTTPS, and will remain so.

[legoktm]

This was done in #5911 / b8ef5d6.