Updates to kube-proxy backend support

[danwinship]

Enhancement Description

• One-line enhancement description (can be used as a release note): Post-KEP-3866, figure out what to do about "default" kube-proxy backend and deprecated backends
• Kubernetes Enhancement Proposal: WIP: KEP-5343: Updates to kube-proxy-backends #5344
• Discussion Link: https://github.com/kubernetes/enhancements/blob/master/keps/sig-network/3866-nftables-proxy/README.md#goals
• Primary contact (assignee): @danwinship
• Responsible SIGs: sig-network
• Enhancement target (which target equals to which milestone):
  • Alpha release target (x.y):
  • Beta release target (x.y):
  • Stable release target (x.y):
• Alpha
  • KEP (k/enhancements) update PR(s):
  • Code (k/k) update PR(s):
  • Docs (k/website) update PR(s):

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

/sig network