Drop `JettyHttpHandlerAdapter` in favor of `JettyCoreHttpHandlerAdapter` / `ServletHttpHandlerAdapter`

[jhoeller]

Our new JettyCoreHttpHandlerAdapter for Jetty 12 supersedes the Servlet bridge for Jetty in general. That aside, as of Servlet 6.1, there is ByteBuffer support in ServletInputStream/ServletOutputStream out of the box; a Jetty-specific subclass of ServletHttpHandlerAdapter won't be necessary for that purpose anymore.