Update Kubernetes Ingress Documentation to push users to modern paths

[tjons]

The Kubernetes Ingress Page opens by instructing users to use the deprecated annotation on the Ingress resource to set the class. This annotation has been deprecated for some time and the IngressClass resource has been adopted as a replacement. Our docs already cover this, but it is presented as a secondary option and the docs explicitly state "The kubernetes.io/ingress.class annotation is required to tell the Istio gateway controller that it should handle this Ingress, otherwise it will be ignored."

This issue is to clean these docs up, promote the IngressClass method to primary, and move the deprecated annotation to secondary.