Use StandardCharsets.UTF_8 for converting String to bytes (#5372)

Use StandardCharsets.UTF_8 for converting String to bytes to avoid potential encoding error.
diff --git a/pulsar-client-kafka-compat/pulsar-client-kafka_0_8/src/test/java/org/apache/pulsar/client/kafka/test/KafkaProducerSimpleConsumerTest.java b/pulsar-client-kafka-compat/pulsar-client-kafka_0_8/src/test/java/org/apache/pulsar/client/kafka/test/KafkaProducerSimpleConsumerTest.java
index 3010c54..82621fb 100644
--- a/pulsar-client-kafka-compat/pulsar-client-kafka_0_8/src/test/java/org/apache/pulsar/client/kafka/test/KafkaProducerSimpleConsumerTest.java
+++ b/pulsar-client-kafka-compat/pulsar-client-kafka_0_8/src/test/java/org/apache/pulsar/client/kafka/test/KafkaProducerSimpleConsumerTest.java
@@ -23,6 +23,7 @@
 
 import java.io.Serializable;
 import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -170,7 +171,7 @@
 
             byte[] bytes = new byte[payload.limit()];
             payload.get(bytes);
-            received.add(new String(bytes, "UTF-8"));
+            received.add(new String(bytes, StandardCharsets.UTF_8));
         }
         lastOffset -= 1;