Reduce the autopkgtest-lxc gap between our fork and the upstream pipeline

As of today, the stretch and jessie autopkgtest jobs rely on the null virtual backend, instead of lxc, as it is the case for more recent releases, including buster.

This issue is to document the attempt to use lxc for those ELTS releases too. To add autopkgtest-lxc support it would be required to:

• create lxc containers in an autopkgtest-lxc project
• be sure the gitlab runner supports them (especially cgroups v1 for jessie)

A simple try to build stretch and jessie autopkgtest-lxc containers fail: https://salsa.debian.org/santiago/autopkgtest-lxc/-/jobs/5709943#L215. I should make debci point correctly to archive.debian.org.

Regarding the salsa shared gitlab runner, it is currently running on bullseye on a cloud-base image. I need to confirm it cgroup v1 is enabled.