Server OS can be 16.04 or 20.04

[kushaldas]

Status

Ready for review

Description of Changes

Fixes #5603

The Server OS version can be either 16.04 or 20.04

Testing

• CI should be green
• To test on focal, first create a local dummy branch from dev_focal and rebase on top of Updates pytest pluggy testinfra molecule and the universe #5585
• git rebase -i updates_pytest_pluggy_testinfra_molecule_and_the_universe
• BASE_OS=focal securedrop/bin/dev-shell bin/run-test -s -v tests/functional/source_navigation_steps.py

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[rmol]

👍

I did not follow the test plan, as tests/functional/source_navigation_steps.py is passing on dev_focal (which has been rebased onto develop after the merge of #5585) without this fix. The relevant test failing under Focal on that branch is tests/functional/test_source_metadata.py, and this change does get that passing.