Update MySQL example stack

The mysql_native_password plugin is now disabled-by-default (and adminer is in a sad state).

mysql/stack.yml

@@ -5,15 +5,7 @@ services:
 
   db:
     image: mysql
-    # NOTE: use of "mysql_native_password" is not recommended: https://dev.mysql.com/doc/refman/8.0/en/upgrading-from-previous-series.html#upgrade-caching-sha2-password
-    # (this is just an example, not intended to be a production configuration)
-    command: --default-authentication-plugin=mysql_native_password
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: example
-
-  adminer:
-    image: adminer
-    restart: always
-    ports:
-      - 8080:8080
+    # (this is just an example, not intended to be a production configuration)