Upgrade to kotlin 2 and the stable kotlinx metadata lib

[TomJuri]

I tried upgrading my mod to kotlin 2.1.0 today and the remapper fails with this error:

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.IllegalArgumentException: kotlinx-metadata-jvm cannot write metadata for future compiler versions. Requested to write version 2.1.0, but highest known version is 2.0.0

kotlinx-metadata-jvm has become a stable library and is published under org.jetbrains.kotlin:kotlin-metadata-jvm:$kotlin_version
See here: https://github.com/JetBrains/kotlin/blob/master/libraries/kotlinx-metadata/jvm/ReadMe.md

[TomJuri]

A temporary workaround is to add
tasks.compileKotlin.get().compilerOptions.freeCompilerArgs.add("-Xmetadata-version=2.0.0")
to the buildscript.