Migrates server-webflux properties to new namespace

Currently, prefix is spring.cloud.gateway and this is ambiguous now that server-webmvc exists.

spring.cloud.gateway.server.webflux is the new prefix

Fixes gh-3363

spring-cloud-gateway-sample/src/main/resources/application-secureheaders.yml

@@ -7,7 +7,7 @@ test:
 
 spring:
   cloud:
-    gateway:
+    gateway.server.webflux:
       filter:
       default-filters:
       #- PrefixPath=/httpbin

spring-cloud-gateway-sample/src/main/resources/application.yml

@@ -9,7 +9,7 @@ spring:
   jmx:
     enabled: false
   cloud:
-    gateway:
+    gateway.server.webflux:
       default-filters:
       - PrefixPath=/httpbin
       - AddResponseHeader=X-Response-Default-Foo, Default-Bar