Switch to gradle's JVM toolchain stuff

[xian]

[slester87]

I tried these changes on my Mac, but gradlew still fails out on the too-new jdk.

Trying to downrev my machine to java 17 now.

Kotlin does not yet support 22 JDK target, falling back to Kotlin JVM_21 JVM target

> Task :buildSrc:compileKotlin FAILED
Kotlin does not yet support 22 JDK target, falling back to Kotlin JVM_21 JVM target

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':buildSrc:compileKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileJava' (22) and 'compileKotlin' (21).

  Consider using JVM Toolchain: https://kotl.in/gradle/jvm/toolchain
  Learn more about JVM-target validation: https://kotl.in/gradle/jvm/target-validation

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.

BUILD FAILED in 7s
2 actionable tasks: 2 executed

Waiting for changes to input files... (ctrl-d to exit)```