Remove usage of deprecated ServerHttpObservationFilter for WebFlux instrumentation

[bclozel]

Since spring-projects/spring-framework#30013, the ServerHttpObservationFilter is deprecated because its position in the WebFlux stack doesn't allow it to record observations accurately. This is being replaced by an instrumentation at the WebHttpHandlerBuilder level, which now auto-detects the ObservationRegistry bean in the context along others (see spring-projects/spring-framework#31205).

We should update our auto-configuration arrangement here to remove the ServerHttpObservationFilter setup and only rely on the Framework detection in the application context for the instrumentation of the WebHttpHandlerBuilder.