WebClient throws exception if Jetty reactive client is not on classpath

[nstdio]

Using WebClient with jetty client connector throws

java.lang.NoClassDefFoundError: org/eclipse/jetty/reactive/client/ReactiveRequest

when org.eclipse.jetty.http2:jetty-http2-client is on classpath and org.eclipse.jetty:jetty-reactive-httpclient is not on classpath.

[rstoyanchev]

Superseded by #35608.