chore: add minimum Kubernetes version requirement (#141)

Enforce the minimum Kubernetes version in Helm chart, according
to our version support policy.

Author: Jonathan Yu

Chart.yaml

@@ -6,6 +6,13 @@ name: coder
 description: Coder moves developer workspaces to your cloud and centralizes their creation and management.
 appVersion: master
 version: 1.19.0
+# Coder follows the Kubernetes upstream version support policy, and the
+# latest stable release version of Coder supports the previous two minor
+# releases as well as the current release of Kubernetes at time of
+# publication.
+#
+# See: https://coder.com/docs/coder/latest/setup/kubernetes#supported-kubernetes-versions
+kubeVersion: ">= 1.19.0-0"
 home: https://coder.com
 keywords:
   - coder