I'm not familiar with the abbreviation "KDM" used in the rancher/rancher milestones (v2.7-KDM-For-2.6.11) can someone point me to an explanation?

KDM stands for Kontainer Driver Metadata. It’s what Rancher uses under the hood to manage dependencies for different versions of Kubernetes. It’s not something a typical user needs to worry about and that branch is likely for testing purposes only.