There is no point to track more than 3 etcd versions at a time
where each etcd versions maps to a k8s CP version.
It's 3 instead of 2 (k8s CP / kubeadm version skew size) because
there is a period of time where the 3rd version (newest) will
be WIP at k/k master - e.g. at the time of this commit it's 1.31.
Add a unit test to block on this.
Also fixate the min etcd version to 3.5.11.
a04cc020f1