Fix XStream serializer issue in KafkaAutoConfigurationIntegrationTest

Fix XStreamSerializer issue due to the Axon Framework 4.5.5 update

kafka-spring-boot-autoconfigure/src/test/java/org/axonframework/extensions/kafka/autoconfig/KafkaAutoConfigurationTest.java

@@ -454,7 +454,7 @@ protected static class TestConfiguration {
 
         @Bean
         public Serializer eventSerializer() {
-            return XStreamSerializer.builder().build();
+            return XStreamSerializer.defaultSerializer();
         }
 
         @Bean