Fix WebFlux deprecation

[garyrussell]

> Task :spring-integration-webflux:compileJava
/Users/grussell/Development/spring-integration/spring-integration-webflux/src/main/java/org/springframework/integration/webflux/outbound/WebFluxRequestExecutingMessageHandler.java:317: warning: [deprecation] <T>fromObject(T) in BodyInserters has been deprecated
                        inserter = BodyInserters.fromObject(requestBody);
                                                ^
  where T is a type-variable:
    T extends Object declared in method <T>fromObject(T)