fix(system-upgrade): Run kubernetes upgrade only on control-plane

This patch enables the upgrades only on control-plane nodes. Given that
my clusters currently have no worker nodes, this is suffient, in the
future one can consider to add a second plan for worker nodes.

infrastructure/system-upgrades/kubernetes.yaml

@@ -60,6 +60,8 @@ spec:
         operator: In
         values:
           - "35"
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
   serviceAccountName: system-upgrade
   secrets:
     - name: kubernetes-upgrade