longhorn: upgrade volume engines automatically, one at a time

concurrentAutomaticEngineUpgradePerNodeLimit was 0, so volumes stayed on
the engine image of whatever Longhorn version created them: three on
1.7.1, nine on 1.7.2, seven on 1.7.3. Every hop of the upgrade to 1.11.2
needs all volumes on the current engine first.

apps/templates/longhorn-helm.yaml

@@ -23,6 +23,10 @@ spec:
             defaultClassReplicaCount: 1
           preUpgradeChecker:
             jobEnabled: false
+          defaultSettings:
+            # Move volumes to the new engine image after each Longhorn
+            # upgrade, one at a time. At 0 they stayed behind on 1.7.1/1.7.2.
+            concurrentAutomaticEngineUpgradePerNodeLimit: 1
         parameters:
           - name: csi.kubeletRootDir
             value: "/var/snap/microk8s/common/var/lib/kubelet"