Update outdated example that uses `JettyHttpHandlerAdapter` in the reference manual

[sbrannen]

Just a quick note that https://github.com/spring-projects/spring-framework/blob/main/framework-docs/modules/ROOT/pages/web/webflux/reactive-spring.adoc still mentions the handler

Originally posted by @philwebb in #33747

[kssumin]

Hi! I'd like to work on this documentation issue.

I'll update the outdated JettyHttpHandlerAdapter example in the reference manual to reflect current best practices and ensure compatibility with Jetty 12.

My plan is to:

1. Update the example code in the reactive-spring.adoc file
2. Ensure compatibility with Jetty 12 API changes
3. Maintain consistency with other server adapter examples in the documentation

I'll submit a PR with the documentation changes soon.