ci: bump juju to 3.6 + charmcraft to 3.x/stable (#629)

* Bump juju to 3.6/stable
* For uniformity, use charmcraft from 3.x/stable

Ref canonical/bundle-kubeflow#1176

.github/workflows/integrate.yaml

@@ -118,7 +118,7 @@ jobs:
         with:
           provider: microk8s
           channel: 1.29-strict/stable
-          juju-channel: 3.4/stable
+          juju-channel: 3.6/stable
           # Pinned to 3.x/stable due to https://github.com/canonical/charmcraft/issues/1845
           charmcraft-channel: 3.x/stable
 
@@ -160,7 +160,7 @@ jobs:
         with:
           provider: microk8s
           channel: 1.29-strict/stable
-          juju-channel: 3.4/stable
+          juju-channel: 3.6/stable
           # Pinned to 3.x/stable due to https://github.com/canonical/charmcraft/issues/1845
           charmcraft-channel: 3.x/stable
           microk8s-addons: "dns hostpath-storage rbac metallb:10.64.140.43-10.64.140.49"