Replace "just run Postgres" with a more meaningful MySQL set-up in multiple-servers

This project: https://github.com/CodeYourFuture/immersive-go-course/tree/main/projects/multiple-servers skims over the database piece and just says "Run a Postgres"

Let's rework this a little to involve setting up a MySQL from scratch, or maybe giving a broken configured MySQL and have our trainees debug why the MySQL isn't working.