Update the base Ubuntu for CI to 20.04, and remove py 2.7, 3.4

[danielmorell]

Description of the change

Ubuntu 18.04 is now deprecated as a GitHub Action runner. This updates the distribution the job runs on to ubuntu-latest.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

None

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[waltjones]

If I remember correctly, python dependencies tend to be closely coupled with Ubuntu version, and having older pythons in the matrix is easier with older ubuntu. I might try using 20.04 instead of latest, and even then the oldest python versions may still have trouble.

[danielmorell]

The ubuntu-20.04 runner image supports Python 3.6 to 3.11 and Python 2.7 until May 15th, 2023. See https://github.com/actions/runner-images/blob/main/images/linux/Ubuntu2004-Readme.md#python. We may need to drop support for legacy Python versions.