ci: don't test against juju 3.6 yet

Author: dimaqq

.github/workflows/test.yaml

@@ -104,7 +104,12 @@ jobs:
           - "3.3/stable"
           - "3.4/stable"
           - "3.5/stable"
-          - "3.6/beta"
+          # A bunch of tests fail with juju.errors.JujuError: base: ubuntu@15.04/stable
+          # * test_subordinate_units
+          # * test_destroy_unit
+          # * test_ssh
+          # * ...
+          # - "3.6/beta"
     steps:
       - name: Check out code
         uses: actions/checkout@v4