Split MCVE template into 3 modules for Java, Scala, and Kotlin

[lukaseder]

The current MCVE template (https://github.com/jOOQ/jOOQ-mcve) mixes Java, Scala, and Kotlin into a single module. If we had 3 modules, it would be easier for users to provide an MCVE, without having to worry about the languages that are irrelevant to them.

While I'm at it, I'll look into replacing the H2 / Flyway usage by a testcontainers one. We don't need Flyway for the MCVE template, but testcontainers would definitely help set up an MCVE that connects to e.g. PostgreSQL, if it's something PG specific.