[DPE-4976] Run tests on juju 3.6 on a nightly schedule (#173)

## Issue
We would like to run nightly tests with juju 3.6

## Solution
Upgrade to dpw v21.0.0 and add juju 3.6 into the matrix for tests. Also,
upgrade libjuju to ^3.5.2.0

.github/workflows/ci.yaml

@@ -116,11 +116,14 @@ jobs:
             bases-index: 0
           - series: jammy
             bases-index: 1
-        juju-snap-channel:  ["2.9/stable", "3.4/stable"]
+        juju-snap-channel:  ["2.9/stable", "3.4/stable", "3.6/beta"]
         include:
+          - juju-snap-channel: "3.6/beta"
+            agent-version: "3.6-beta2"
+            libjuju-version: "3.5.2.0"
           - juju-snap-channel: "3.4/stable"
             agent-version: "3.4.3"
-            libjuju-version:  "3.4.0.0"
+            libjuju-version:  "3.5.2.0"
           - juju-snap-channel: "2.9/stable"
             agent-version: "2.9.45"
             libjuju-version:  "2.9.44.1"
@@ -168,6 +171,7 @@ jobs:
             echo "mark_expression=not unstable" >> "$GITHUB_OUTPUT"
           fi
       - name: Run integration tests
+        if: ${{ !startsWith(matrix.juju-snap-channel, '3.6/') || github.event_name == 'schedule' }}
         run: tox run -e integration -- "${{ matrix.groups.path_to_test_file }}" --group="${{ matrix.groups.group_number }}" -m '${{ steps.select-test-stability.outputs.mark_expression }}' --mysql-router-charm-series=${{ matrix.ubuntu-versions.series }} --mysql-router-charm-bases-index=${{ matrix.ubuntu-versions.bases-index }}
         env:
           LIBJUJU_VERSION_SPECIFIER: ${{ matrix.libjuju-version }}