Introduced in https://github.com/jenkinsci/jenkins/pull/6694

This line returns false since Jetty 10 upgrade:

https://github.com/jenkinsci/jenkins/blob/76e5c98dd3c92ee892c7d094d246602be2e1dc79/core/src/main/java/jenkins/websocket/WebSockets.java#L89

Because it fails to load the required Jetty classes, there's been at least package name changes if not more.

Steps to reproduce:
1. Create an agent with websocket checkbox enabled
2. Form validation will reply with "WebSocket support is not enabled in this Jenkins installation"

If you enable a FINE log recorder for `jenkins.websocket.WebSockets` you get a class not found exception