Don't use JUnit 5

https://build.spring.io/browse/SK-SK20X-45/

Author: artembilan

spring-kafka/src/test/java/org/springframework/kafka/core/DefaultKafkaProducerFactoryTests.java

@@ -28,7 +28,7 @@
 
 import org.apache.kafka.clients.producer.Producer;
 import org.apache.kafka.common.KafkaException;
-import org.junit.jupiter.api.Test;
+import org.junit.Test;
 import org.mockito.InOrder;
 
 import org.springframework.kafka.test.utils.KafkaTestUtils;