Page to review

What would you like to be added
https://kubernetes.io/docs/tasks/administer-cluster/securing-a-cluster/ mentions Kubernetes v1.7
Review this page, removing references to Kubernetes versions that are earlier than what's supported, and revising wording to suit.

Why is this needed
The documentation usually can and should assume that the reader is running a version of Kubernetes that is at least as new as the oldest version still supported.

Comments
/kind cleanup