[PSA] Recommended Ubuntu LTS changing to 20.04

[Jack Rosenthal]

Hi folks,

While ChromiumOS can be built on a large variety of Linux distributions, our developer guide has recommended Ubuntu 18.04 LTS ("Bionic") if you weren't quite sure which distro to choose from.

This must change as:

• Standard support for Ubuntu 18.04 Bionic ends April 2023.
• We intend to require Python 3.8+ in the host environment soon, and Ubuntu 18.04 ships with Python 3.6 by default.

This CL updates the recommended Ubuntu version from 18.04 Bionic to 20.04 LTS ("Focal").

If you're using Ubuntu 18.04, here are instructions on how to upgrade using the GUI or using the command line.

Thanks,

Jack