Run juju 3.6 nightly tests against 3.6/stable (#337)

## Issue
Juju 3.6/stable was released

## Solution
Run juju 3.6 nightly tests against 3.6/stable

Author: shayancanonical

.github/workflows/ci.yaml

@@ -78,7 +78,7 @@ jobs:
             allure_on_amd64: false
           - agent: 3.5.4  # renovate: juju-agent-pin-minor
             allure_on_amd64: true
-          - snap_channel: 3.6/candidate
+          - snap_channel: 3.6/stable
             allure_on_amd64: false
         architecture:
           - amd64
@@ -88,7 +88,7 @@ jobs:
               allure_on_amd64: true
             architecture: arm64
           - juju:
-              snap_channel: 3.6/candidate
+              snap_channel: 3.6/stable
               allure_on_amd64: false
             architecture: arm64
     name: Integration | ${{ matrix.juju.agent || matrix.juju.snap_channel }} | ${{ matrix.architecture }}