[bn] Upgrade mysql image version used by Single-Instance Stateful tutorial

Author: serut

content/bn/examples/application/mysql/mysql-deployment.yaml

@@ -25,7 +25,7 @@ spec:
         app: mysql
     spec:
       containers:
-      - image: mysql:5.6
+      - image: mysql:9
         name: mysql
         env:
           # Use secret in real usage