Update minimum supported kubernetes version to 1.13

1.12 is no longer supported by all our cloud targets.

• 1.12 has being dropped from GKE in February https://cloud.google.com/kubernetes-engine/docs/release-notes#february_4_2020
• 1.13 has already been dropped from GKE in March: https://cloud.google.com/kubernetes-engine/docs/release-notes#march_20_2020
• 1.12 is being dropped from EKS on May 11th: https://docs.aws.amazon.com/eks/latest/userguide/kubernetes-versions.html#version-deprecation

We should plan to update our documentation to reference 1.13 as the minimum supported version, and update all our tooling to use 1.14 versions of the kubernetes libraries. (The libraries and apis offer version compatibility back one version)

Previous update issue: #1583 (closed)