deprecate and drop cgroup v1 support

[giuseppe]

opening an issue to make clear the idea of deprecating support for cgroup v1, and eventually dropping it.

cgroup v2 has been available for many years now, it is more secure and allows being used in a rootless environment. Continuing to support cgroup v1 is no longer necessary.

Optimistically we could aim for deprecation in the next 6 months and removal in 12-18 months.

[rhatdan]

I would want to line this up with RHEL 8 going into the next phase, which I think happens in the fall.

[rhatdan]

But depracating this now makes sense.