Make UndertowAccessLogCustomizer conditional on class

Closes gh-15065

Author: snicoll

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/servlet/ServletManagementChildContextConfiguration.java

@@ -70,6 +70,7 @@ public ServletManagementWebServerFactoryCustomizer servletManagementWebServerFac
 	}
 
 	@Bean
+	@ConditionalOnClass(name = "io.undertow.Undertow")
 	public UndertowAccessLogCustomizer undertowAccessLogCustomizer() {
 		return new UndertowAccessLogCustomizer();
 	}