Move cgroup v1 support into maintenance mode

[harche]

Enhancement Description

• One-line enhancement description (can be used as a release note): Moving cgroup v1 support into maintenance mode
• Kubernetes Enhancement Proposal: https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/4569-cgroup-v1-maintenance-mode/README.md
• Discussion Link:
• Primary contact (assignee): @harche
• Responsible SIGs: sig-node
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y):
  • Beta release target (x.y):
  • Stable release target (x.y): 1.31
• [x] Stable
  • [x] KEP (k/enhancements) update PR(s): https://github.com/kubernetes/enhancements/pull/4572
  • [x] Code (k/k) update PR(s):
    • [x] https://github.com/kubernetes/kubernetes/pull/125496
    • [x] https://github.com/kubernetes/kubernetes/pull/125328
    • [x] https://github.com/kubernetes/kubernetes/pull/126031
  • [x] Docs (k/website) update PR(s):
    • [x] https://github.com/kubernetes/website/pull/47069

Please keep this description up to date. This will help the Enhancement Team to track the evolution of the enhancement efficiently.