Skip Jaxb2RootElementHttpMessageConverter auto-detection in HttpMessageConverters

[bclozel]

Currently, the Jaxb2RootElementHttpMessageConverter is auto-detected by HttpMessageConverters when the registerDefaults() option is enabled (as it's always the case for the MVC infrastructure).

We received consistent feedback, saying that:

1. classpath detection is not really useful because jakarta.xml.bind.Binder is often brought transitively at runtime (for example, hibernate-core)
2. the nature of that message converter requires extensive runtime checks to know whether it should be used

As a result, we should disable its auto-detection and let users register it manually if they intend to use it in their applications.

In a typical Spring application, this is as a easy as:

@Configuration(proxyBeanMethods = false)
class JaxbConverterConfiguration implements WebMvcConfigurer {

	@Override
	public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder) {
		builder.withXmlConverter(new Jaxb2RootElementHttpMessageConverter());
	}
}