Drop use of OutputCapture in SampleKafkaApplicationTests

Closes gh-15432

spring-boot-samples/spring-boot-sample-kafka/src/main/java/sample/kafka/Consumer.java

@@ -16,15 +16,25 @@
 
 package sample.kafka;
 
+import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
+
 import org.springframework.kafka.annotation.KafkaListener;
 import org.springframework.stereotype.Component;
 
 @Component
 class Consumer {
 
+	private final List<SampleMessage> messages = new CopyOnWriteArrayList<>();
+
 	@KafkaListener(topics = "testTopic")
 	public void processMessage(SampleMessage message) {
+		this.messages.add(message);
 		System.out.println("Received sample message [" + message + "]");
 	}
 
+	List<SampleMessage> getMessages() {
+		return this.messages;
+	}
+
 }