Update mysql Docker tag to v9

Author: renovate[bot]

docker-compose.yml

@@ -32,7 +32,7 @@ services:
 
   gnuboard-db:
     restart: always
-    image: mysql:8.0
+    image: mysql:9.6
     volumes:
       - db_data:/var/lib/mysql
       - db_socket:/var/lib/mysqld