[Bug] In the example dubco-samples-spring-boot-trace-skywalking, the run fails about dubbo

[AlbumenJ]

@conghuhu @songxiaosheng PTAL

[songxiaosheng]

@xiaozhigang PATL

[xiaozhigang]

your jdk:1.8
requirement：jdk-17

[glynnwei]

your jdk:1.8 requirement：jdk-17

Support 1.8, do not change 17
org.apache.dubbo.springboot.skywalking.demo.provider.ObservationConfiguration add fix code:

@ConditionalOnBean(ObservationRegistry.class)
@Bean
ApplicationModel applicationModel(@Autowired ObservationRegistry observationRegistry) {
    ApplicationModel applicationModel = ApplicationModel.defaultModel();
    observationRegistry.observationConfig()
            .observationHandler(new ObservationHandler.FirstMatchingCompositeObservationHandler(
                    new SkywalkingSenderTracingHandler(), new SkywalkingReceiverTracingHandler(),
                    new SkywalkingDefaultTracingHandler()
            ));
    applicationModel.getBeanFactory().registerBean(observationRegistry);
    return applicationModel;
}