KafkaAdmin: fix usage of the Map.of()

* Fix `KafkaAdmin` to use `Collections.singletonMap()` instead of
`Map.of()` which is not available for Java 8 yet

Author: garyrussell

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2021 the original author or authors.
+ * Copyright 2017-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -342,8 +342,9 @@ private void adjustConfigMismatches(AdminClient adminClient, Collection<NewTopic
 					}
 					if (alterConfigOperations.size() > 0) {
 						try {
-							AlterConfigsResult alterConfigsResult = adminClient
-									.incrementalAlterConfigs(Map.of(topicConfigResource, alterConfigOperations));
+							AlterConfigsResult alterConfigsResult =
+									adminClient.incrementalAlterConfigs(
+											Collections.singletonMap(topicConfigResource, alterConfigOperations));
 							alterConfigsResult.all().get(this.operationTimeout, TimeUnit.SECONDS);
 						}
 						catch (InterruptedException ie) {