* JRuby 10 requires Java 21. Since the previous default was 17 and all JRuby releases should work fine on 21, we do this for all JRuby installs.
* Implements #718

Apparently this exec errors when the subprocess fails rather than
returning an error code. This causes the whole setup process to
terminate. This patch catches the error and uses that to indicate
failure to launch.

Native launcher appears to hang on GHA when the JDK version is too
old for JRuby 10. This bypasses it and should be a bit lighter. It
will also avoid any variability in the launcher used, since Unixes
and Windows already use different executables.