Fixes https://github.com/spring-projects/spring-integration/issues/2358 The WebFlux on the server side can deal with MVC `@Controllers` as well without any Servlet environment. * Enable `IntegrationGraphController` when we not only in the Servlet environment, but also in the WebFlux * Upgrade to SF-5.0.4 and fix WebFlux tests to meet the latest requirements (https://jira.spring.io/browse/SPR-16337) * Move `WEB_FLUX_PRESENT` to the `HttpContextUtils`; deprecate it in the `WebFluxContextUtils` * Mention WebFlux ability for the `IntegrationGraphController` in the `graph.adoc`