Adapt test to changes in RSocket Java

See gh-24934

spring-messaging/src/test/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterBuilderTests.java

@@ -279,6 +279,11 @@ public class DefaultRSocketRequesterBuilderTests {
 			return Flux.empty();
 		}
 
+		@Override
+		public ByteBufAllocator alloc() {
+			return ByteBufAllocator.DEFAULT;
+		}
+
 		@Override
 		public Mono<Void> onClose() {
 			return Mono.empty();