Update the support end date for cluster k8s agents

The documentation in https://docs.gitlab.com/ee/user/clusters/agent/#supported-kubernetes-versions-for-gitlab-features sets the "support ends" for Kubernetes versions to a milestone date (previously the 22nd of a given month). Since we are changing the milestone release dates to every 3rd Thursday in a month, we should also change the support end dates according to the milestone.

Update the Kubernetes version "support ends" documentation to be by milestone instead of by a specific date, e.g.:

support ends on July 22, 2024 or when 1.30 becomes supported

becomes

support ends when GitLab version 17.4 is released or when 1.30 becomes supported