ci: integration tests against multiple juju versions

Author: dimaqq

.github/workflows/test.yaml

@@ -95,6 +95,9 @@ jobs:
           # save some resources for now.
           # - "3.9"
           - "3.10"
+        juju:
+          - "3.1/stable"
+          - "3.3/stable"
     steps:
       - name: Check out code
         uses: actions/checkout@v4