KEP-3935: support oldest nodes with newest control planes

[Jordan Liggitt]

Hello all,

I've opened KEP-3935 with a proposal to expand supported node/control-plane skew from n-2 to n-3 versions, and wanted to get eyes on it.

There are plenty of details in the PR, but the short version is it brings the skew policy back in line with its original goal of letting oldest supported nodes work with newest supported control planes, even during the 2-month overlap after a new Kubernetes minor release before the oldest minor reaches end-of-life. There are examples in the KEP of how this can streamline upgrades and simplify qualification for users.

I've added it to today's sig-arch agenda for discussion, and I'm also making the rounds of other relevant sigs to get feedback, but wanted to signal boost it here.

Thanks,

Jordan