Fix Sonar Issue

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/core/KafkaAdmin.java

@@ -338,7 +338,7 @@ private void modifyTopics(AdminClient adminClient, Map<String, NewPartitions> to
 	 */
 	public static class NewTopics {
 
-		final Collection<NewTopic> newTopics = new ArrayList<>();
+		private final Collection<NewTopic> newTopics = new ArrayList<>();
 
 		/**
 		 * Construct an instance with the {@link NewTopic}s.