Release notes for kOps 1.20 series

(The kOps 1.20 release has not been released yet; this is a document to gather the notes prior to the release).

Significant changes

Added template funtions for kubernetes version based on channel data.
kOps now use helm3 functions for merging template --set and --values arguments. This has slightly different behaviour than previous helm2-like logic.
Following kubeadm, control plane nodes are now labelled with node-role.kubernetes.io/control-plane=""
Default node image for GCE changed from COS to Ubuntu for K8s versions >= 1.17.0. This is to more closely align with the AWS implementation (the most mature support) and because COS limits the ability to modify files on its disk.

Support for Kubernetes 1.11 and 1.12 has been removed.
Support for Terraform version 0.11 has been removed.
Support for the feature flag Terraform-0.12 has been removed. All generated Terraform HCL2/JSON files will support versions 0.12.26+ and 0.13.0+.

If you are running kops toolbox template in an airgapped environment, you have to set --channel to point to a local channel file.
If your workload targets control plane nodes, you need to change them to select the node-role.kubernetes.io/control-plane="" label. You should also add the node-role.kubernetes.io/control-plane:NoSchedule toleration to these workloads. This taint will not be added to control plane nodes before kOps 1.22.

Support for Kubernetes versions 1.13 and 1.14 are deprecated and will be removed in kOps 1.21.
The manifest based metrics server addon has been deprecated in favour of a configurable addon.
The manifest based cluster autoscaler addon has been deprecated in favour of a configurable addon.
The node-role.kubernetes.io/master and kubernetes.io/role labels are deprecated and will be removed from control plane nodes in kOps 1.22