build: Switch to ubuntu-latest for builds

[feanil]

This code does not have any dependencies that are specific to any specific version of ubuntu. So instead of testing on a specific version and then needing to do work to keep the versions up-to-date, we switch to the ubuntu-latest target which should be sufficient for testing purposes.

This work is being done as a part of https://github.com/openedx/platform-roadmap/issues/377

closes https://github.com/openedx/auth-backends/issues/330