Improve Jetty 10 check in JettyClientHttpResponse

[poutsma]

JettyClientHttpResponse checks for the presence of org.eclipse.jetty.websocket.server.JettyWebSocketServerContainer to determine whether it is running on Jetty 10, but that type is not necessarily present when using just the client libraries, without Jetty server.

We should improve JettyClientHttpResponse's Jetty 10 check.

[poutsma]

Note that the previous commit linked to this issue by mistake. It should have pointed to #27117