ci: juju 3.6.0 is stable

Author: dimaqq

.github/workflows/test.yaml

@@ -77,7 +77,7 @@ jobs:
           - "3.3/stable"
           - "3.4/stable"
           - "3.5/stable"
-          - "3.6/candidate"
+          - "3.6/stable"
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
@@ -133,6 +133,7 @@ jobs:
           - "3.3/stable"
           - "3.4/stable"
           - "3.5/stable"
+          - "3.6/stable"
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5