Remove all Xenial-specific code from the codebase

[eloquence]

SecureDrop 2.0.0 will be the first release to no longer support Ubuntu 16.04 (Xenial). As part of that release (but no sooner), we should remove all Xenial-specific code from the codebase, including any Xenial-specific application code, provisioning logic, or developer tooling.

Next steps

• Update Vagrantfile
• Remove the following molecule scenarios
• builder-xenial, libvirt-staging-xenial, qubes-staging-xenial, virtualbox-staging-xenial
• Update make build-debs to make build-debs-focal and remove the second
• Update make build-debs-notest to make build-debs-notest-focal and remove the second
• Update make staging to same of make staging-focal and remove the second
• Update make dev to same of make dev-focal and remove the second
• Update ci-deb-tests to ci-deb-tests-focal and remove the second
• Update CI to remove all Xenial based jobs, make sure that all scripts have focal as default
• Next, look at the source and journalist code to find all Python3.5 related points

[eloquence]

Suggestions for how this work could potentially be divided up would be appreciated ahead of Thursday's sprint planning discussion. One potential division @emkll and I discussed is:

• packaging / packaging tests
• provisioning / provisioning tests
• CI

[eloquence]

This is one of our top 3 priorities for the 4/15 sprint, with @kushaldas @creviera and @zenmonkeykstop collaborating on this and the v2 onion service removal (#5731).