Add gradle with Kotlin DSL for Java 16 fix guidance

[Christian Hausknecht]

First of all I appreciate the fix guidance for Java 16 builds - it works and solves the testing troubles.

I would suggest to add the fix also for users that rely on the Kotlin gradle DSL:

if (JavaVersion.current() >= JavaVersion.VERSION_16) {
    tasks.withType<Test>().all {
        jvmArgs(
            "--add-opens=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.jvm=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED",
            "--add-opens=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED",
        )
    }
}

As this is a Kotlin library, I would assume most users will use their gradle with Kotlin DSL anyways :-)