switch k3s to latest release channel

fix for [0] and [1] that kept me on 1.18 were released as part of 1.20

also mount dir where certs are symlinked to, trying to fix [2]

[0]: https://github.com/kubernetes/kubernetes/issues/94335
[1]: https://github.com/kubernetes/kubernetes/issues/95826
[2]: https://github.com/rancher/system-upgrade-controller/issues/105

plans/plans.yaml

@@ -16,4 +16,4 @@ spec:
   serviceAccountName: system-upgrade
   upgrade:
     image: rancher/k3s-upgrade
-  version: v1.18.9+k3s1
+  channel: https://update.k3s.io/v1-release/channels/latest

plans/system-upgrade-controller.yaml

@@ -82,6 +82,8 @@ spec:
           name: etc-ssl
         - mountPath: /tmp
           name: tmp
+        - mountPath: /etc/ca-certificates/extracted
+          name: etc-extracted
       serviceAccountName: system-upgrade
       tolerations:
       - key: CriticalAddonsOnly
@@ -96,3 +98,7 @@ spec:
         name: etc-ssl
       - emptyDir: {}
         name: tmp
+      - hostPath:
+          path: /etc/ca-certificates/extracted
+          type: Directory
+        name: etc-extracted