[FLINK-37644] Remove jetbrains annotations

Author: AHeise

flink-connector-kafka/src/test/java/org/apache/flink/connector/kafka/sink/KafkaSinkTest.java

@@ -16,10 +16,11 @@
 import org.apache.flink.streaming.api.lineage.LineageVertex;
 
 import org.apache.kafka.clients.producer.ProducerRecord;
-import org.jetbrains.annotations.Nullable;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
+import javax.annotation.Nullable;
+
 import java.util.Collections;
 import java.util.Optional;
 import java.util.Properties;