SpringBoot Re-enable Jetty 10 h2c testing once Jetty 10 and Apache HttpClient 5 incompatibility has been addressed

See https://github.com/eclipse/jetty.project/issues/6164.

Comment From: wilkinsona

The Jetty team have identified that it's an Apache HttpClient 5 problem. I've opened HTTPCORE-673.

Comment From: wilkinsona

There's an Undertow test that's flaky when using HttpClient 5 so I'm tempted to switch to a different client. Some local experimentation suggests that Jetty's HTTP/2 client is more robust so I'm going to try switching to that.