Fix MySQL Docker container in Koa test app

The MySQL database container failed to start. It failed with this error:

> unknown variable 'default-authentication-plugin=mysql_native_password'

This is due to the untagged `mysql` image being updated to MySQL 8.4.

Replacing it with `--mysql-native-password=ON`, as described in the
[MySQL 8.4 changelog][changelog], runs into a different issue, where
the `mysql` npm package does not support the newer MySQL authentication
mechanisms.

The `mysql` npm package is de facto deprecated in lieu of the `mysql2`
npm package. There has been an outstanding [open PR][pr] for the last
five years, which is very unlikely to be merged.

This commit locks the MySQL version to 8.3, which is the last version
that still supports the now deprecated authentication mechanisms, and
therefore the last version that will be compatible with the `mysql`
npm package.

[skip changeset]

[changelog]: https://dev.mysql.com/doc/refman/8.4/en/mysql-nutshell.html
[pr]: mysqljs/mysql#2233

Co-authored-by: Noemi Lapresta <noemi@appsignal.com>

test/koa-mysql/docker-compose.yml

@@ -2,7 +2,7 @@ version: "3.8"
 
 services:
   mysql:
-    image: mysql
+    image: mysql:8.3
     command: --default-authentication-plugin=mysql_native_password
     restart: always
     env_file: