Update dependency Juju 3 to v3.1.8 (#436)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yaml

@@ -54,7 +54,7 @@ jobs:
           - agent: 2.9.49  # renovate: latest juju 2
             libjuju: ==2.9.46.1  # renovate: latest libjuju 2
             allure: false
-          - agent: 3.1.7  # renovate: latest juju 3
+          - agent: 3.1.8  # renovate: latest juju 3
             allure: true
     name: Integration test charm | ${{ matrix.juju.agent }}
     needs: