Change @ConditionalOnClass from Servlet to Flux

Update `ReactiveManagementContextAutoConfiguration` to check for a
`Flux` class rather than `Servlet`.

Closes gh-10590

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/web/reactive/ReactiveManagementContextAutoConfiguration.java

@@ -16,7 +16,7 @@
 
 package org.springframework.boot.actuate.autoconfigure.web.reactive;
 
-import javax.servlet.Servlet;
+import reactor.core.publisher.Flux;
 
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
@@ -33,7 +33,7 @@ import org.springframework.context.annotation.Configuration;
  * @since 2.0.0
  */
 @Configuration
-@ConditionalOnClass(Servlet.class)
+@ConditionalOnClass(Flux.class)
 @ConditionalOnWebApplication(type = Type.REACTIVE)
 public class ReactiveManagementContextAutoConfiguration {