JDK 15 is removing their Javascript evaluation engine, which is used by the thread pool size Javascript evaluation code. The Javascript option isn't used much by the community, so we should remove it so OptaPlanner will continue to work on JDK 15+ without adding another dependency.

This is where various options were discussed: https://docs.google.com/document/d/1rHYaAVYlMG7Cd6sS4k11xyv-M6nBs7gOSWK5HaukE3U/edit 

This is an example of what happens on JDK 15 when you try to execute a JavaScript expression from one of our tests:

java.lang.IllegalStateException: The moveThreadCount (-1) could not resolve because the JVM doesn't support scriptLanguage (JavaScript).