Update Gradle configuration to Kotlin DSL

[retrodaredevil]

I've been using Gradle DSL for a while, but I think it's finally time to upgrade SolarThing to use Kotlin DSL, as I believe it has matured in recent years (I'm especially impressed by what gradle init produces when choosing Kotlin DSL).

This issue serves to document progress in migrating all of SolarThing's Gradle configuration to use Kotlin DSL.

https://docs.gradle.org/current/kotlin-dsl/index.html