update vttestserver mysql version doc

Signed-off-by: Harshit Gangal <harshit@planetscale.com>

content/en/docs/16.0/get-started/vttestserver-docker-image.md

@@ -58,7 +58,7 @@ The docker image expects some of the environment variables to be set to function
 | *PORT* | yes | The starting of the port addresses that vitess will use to register its components like vtgate, etc. |
 | *MYSQL_MAX_CONNECTIONS* | no | Maximum number of connections that the MySQL instance will support. If unspecified, it defaults to 1000. |
 | *MYSQL_BIND_HOST* | no | Which host to bind the MySQL listener to. If unspecified, it defaults to `127.0.0.1`. |
-| *MYSQL_SERVER_VERSION* | no | MySQL server version to advertise. If unspecified, it defaults to `8.0.21-vitess` or `5.7.9-vitess` according to the version of vttestserver run. |
+| *MYSQL_SERVER_VERSION* | no | MySQL server version to advertise. If unspecified, it defaults to `8.0.31-vitess` or `5.7.9-vitess` according to the version of vttestserver run. |
 | *CHARSET* | no | Default charset to use. If unspecified, it defaults to `utf8mb4`. |
 | *FOREIGN_KEY_MODE* | no | This is to provide how to handle foreign key constraint in create/alter table. Valid values are: allow (default), disallow. |
 | *ENABLE_ONLINE_DDL* | no | Allow users to submit, review and control Online DDL. Valid values are: true (default), false. |