test: run microk8s against 3 1 rather than 2 9

Charms we need to test with microk8s do not support 2.9, thus test
against 3.1 for legacy code paths. Juju 3.1. requires the strictly
confined microk8s snap.

Update lxd to 5.21 channel while here.

.github/workflows/test_integration.yml

@@ -47,10 +47,10 @@ jobs:
       matrix:
         terraform: ["1.6.*", "1.7.*", "1.8.*"]
         action-operator:
-          - { lxd-channel: "5.19/stable", cloud: "lxd", cloud-channel: "5.19", juju: "2.9" }
-          - { lxd-channel: "5.19/stable", cloud: "lxd", cloud-channel: "5.19", juju: "3" }
-          - { lxd-channel: "5.19/stable", cloud: "microk8s", cloud-channel: "1.28", juju: "2.9" }
-          - { lxd-channel: "5.19/stable", cloud: "microk8s", cloud-channel: "1.28-strict", juju: "3" }
+          - { lxd-channel: "5.21/stable", cloud: "lxd", cloud-channel: "5.21", juju: "2.9" }
+          - { lxd-channel: "5.21/stable", cloud: "lxd", cloud-channel: "5.21", juju: "3" }
+          - { lxd-channel: "5.21/stable", cloud: "microk8s", cloud-channel: "1.28-strict", juju: "3.1" }
+          - { lxd-channel: "5.21/stable", cloud: "microk8s", cloud-channel: "1.28-strict", juju: "3" }
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v4