Test cgroups v2 in CI

[PhilippWendler]

Right now, all our CI tests are executed on a special GitLab runner configured with cgroups v1 and a privileged Docker container that allows cgroup access. Now we want to additionally test BenchExec with cgroups v2.

Unfortunately, Docker does not support proper delegation of cgroups v2 into a container, only via the crude hack of giving full access to the cgroup hiearchy (like for cgroups v1), so we would need yet another special privileged runner.

A better solution would be using Podman for the runner, so we could just use our standard non-privileged runners. However, GitLab requires at least Podman 4.2, which we currently do not have.