Move next-devel to cgroupsv2 for new nodes

All we need to do is to make the associated karg be specific to
`testing-devel` by uplisting it to `image.yaml`. Then `next-devel` will
naturally shed it when `image-base.yaml` gets synced over.

See: coreos/fedora-coreos-tracker#292

image.yaml

@@ -2,3 +2,8 @@
 # similarly to manifest.yaml. Unlike image-base.yaml, which is shared by all
 # streams.
 include: image-base.yaml
+
+extra-kargs:
+    # https://github.com/coreos/fedora-coreos-tracker/issues/292
+    # https://fedoraproject.org/wiki/Changes/CGroupsV2
+    - systemd.unified_cgroup_hierarchy=0