Update build/CI automation to use ubuntu-20.04

[philrz]

We've been using the ubuntu-18.04 platform for some time to test and build Brim. Recent notifications from GitHub have indicated:

The official deprecation notice is here.

In reaction to this, I plan to:

1. Create test builds using the ubuntu-20.04 Actions Runners
2. Smoke test it on older Linux releases
3. Make any necessary updates to our support statement at https://github.com/brimdata/brim/wiki/Supported-Platforms
4. Advance our Actions Workflow YAML configs to start using the ubuntu-20.04 Actions Runners going forward