Documentation update

[sergey-morenets]

Describe the bug

The current documentation (https://docs.spring.io/spring-cloud-sleuth/docs/current/reference/html/project-features.html#project-features) contains incorrect or out-of-date code samples.

Sample

@Configuration(proxyBeanMethods = false)
    class MyConfig {
    @Bean ZipkinRestTemplateCustomizer myCustomizer() {
        return new ZipkinRestTemplateCustomizer() {
            @Override
            void customize(RestTemplate restTemplate) {
                // customize the RestTemplate
            }
        };
    }
}

It seems that this method was deprecated in 2.2.0 and removed in the next versions. New method was added instead:

	default RestTemplate customizeTemplate(RestTemplate restTemplate) {
		return restTemplate;
	}