[all][qa] Devstack dropping support for Ubuntu Focal

Hello Everyone, As you know, this release 2023.2 (bobcat) does not mandate testing the ubuntu Focal[1] and with Nova bumped the libvirt version, focal jobs are failing. Devstack was planning to drop the Focal support in the next cycle but we have to do it now only[2]. This email is a heads-up if you are still wondering about the Focal job failure. If your project still running any of the jobs on focal, please plan to move to Jammy. [1] https://governance.openstack.org/tc/reference/runtimes/2023.2.html [2] https://review.opendev.org/c/openstack/devstack/+/885468 -gmann