Review target compileJava15Java in btrace-runtime/build.gradle

[magicwerk]

Target compileJava15Java seems to be a strange mixture of Java versions 15, 11, and 17...

compileJava15Java {
    sourceCompatibility = 11
    targetCompatibility = 11
    options.fork = true
    options.forkOptions.executable = "${System.getenv("JAVA_17_HOME")}/bin/javac.exe"
    options.forkOptions.javaHome = file("${System.getenv("JAVA_17_HOME")}")
    options.compilerArgs.addAll(['--add-exports', 'java.base/jdk.internal.reflect=ALL-UNNAMED', '--add-exports', 'java.base/jdk.internal.perf=ALL-UNNAMED'])
}