Add support for Kubernetes versions 1.17, 1.18, 1.19

Notes

This is an umbrella issue primarily for the release note as we tackle it together for Kubernetes 1.17, 1.18, 1.19

Release notes

By adding support to more recent Kubernetes version GitLab users can benefit from GitLab's various Kubernetes integrations, like the GitLab Kubernetes Agent, Auto DevOps and GitLab Managed Apps even on more recent clusters. Previously official support was provided only up to Kubernetes version 1.16. You can find the list of supported versions and related timelines in our documentation.

https://docs.gitlab.com/ee/user/project/clusters/#supported-cluster-versions