Move to Scala 3

[robons]

We're currently using Scala 2.1x.y but Scala 3 has been released. It's a bit of a step-change so it'd be good to spend some time figuring out what code we need to change, what will break, etc.

Then we should convert the code-base to Scala 3 so we don't fall too far behind with the functionality available & security holes, etc.