Use Gradle's toolchain management instead of your own

[doctorpangloss]

Allow the user to specify a toolchain using Gradle's built-in functionality in 8+ and retire your current toolchain management approach.

settings.gradle:

plugins {
  id('org.gradle.toolchains.foojay-resolver-convention') version '0.6.0'
}

build.gradle:

java {
  sourceCompatibility = JavaVersion.VERSION_20
  toolchain {
    vendor = JvmVendorSpec.GRAAL_VM
    languageVersion = JavaLanguageVersion.of(20)
  }
}

At this stage, the toolchain will be downloaded and discoverable.