Due to a few changes in `WebSocketServerFactory` and `Session` API, our
`JettyRequestUpgradeStrategy` and `JettyWebSocketSession` needed to
adapt. As of 9.3.15+ and 9.4.0+, some reflection is required to support
previous versions.

Spring 5.0 supports Jetty 9.3 and 9.4.

Issue: SPR-14940