Add Scala 2.13 build target.

[megri]

Heya! It'd be nice if we could build for Scala 2.13. It would enable interoperability with Scala 3 and a lot of other goodies. WDYT?

[pflooky]

Hey @megri, thanks for raising this. I think it would be great to use Scala 2.13. It might be a bit tricky with dependencies but should technically work. Would you like to try create a PR for it?

[megri]

Sure, I'll see what I can do! On a side-note how married are you to gradle? Would you be ok with migrating to mill if I run into problems? It's been a while since I tried working with gradle.

[pflooky]

I would like to keep gradle as is. You can first try using the gradle wrapper via './gradlew which is used for all the github action builds, etc.