fix: update jetty to supported version

[cortlepp]

see also #946

[niloc132]

If you're going to change this, you must also change maven.compiler.source etc to be 17+.

Instead, consider a fork/update of htmlunitwebsocketclient that uses newer jetty, or switches to okhttp or the like, so that your downstream project isn't dependent on Jetty 9. Note that two of the dependencies changed here are scope=test, so won't impact your own project.