fix(quickstart-gateway-service): fix route failure caused by Spring Cloud Gateway 5.x property prefix change

Spring Cloud Gateway 5.0.1 (webflux) renamed the configuration prefix from
spring.cloud.gateway.* to spring.cloud.gateway.server.webflux.*, causing the
discovery.locator and routes configuration to be ignored, so the gateway
could not forward requests (returned 404).

spring-cloud-tencent-examples/quickstart-example/quickstart-gateway-service/src/main/resources/application.yml

@@ -33,6 +33,8 @@ spring:
       stat:
         enabled: true
     gateway:
+      server:
+        webflux:
           discovery:
             locator:
               enabled: true