Update GitHub Actions template and documentation to specify Ubuntu version

[hancush]

Documentation request

We currently use ubuntu-latest, but this does not always match our server environments, especially for legacy deployments. We should update our cookiecutter to allow a version to be set and update the docs for migrating legacy apps to GitHub Actions to consult the EC2 console and use the Ubuntu version running on the server.

Docs on runs-on: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idruns-on

[hancush]

I will add sentence to migration docs on this key, as well as a note in the template.