Change charm channel in bundle for test

Author: cderici

tests/integration/bundle/test-overlays/bundle-with-overlay-multi.yaml

@@ -5,7 +5,7 @@ applications:
     num_units: 1
   mysql:
     charm: "mysql"
-    channel: candidate
+    channel: 8.0/stable
     num_units: 1
 relations:
   - ["ghost", "mysql"]