GH-1693: Use concatMap in ReactiveKafkaTemplate

Fixes #1693

The documentation of the `reactor-kafka` project states that messages should be consumed 
using the `concatMap` operator, but the `ReactiveKafkaConsumerTemplate` uses the `flatMap`
operator at least in case of the `receiveAutoAck` method.

spring-kafka/src/main/java/org/springframework/kafka/core/reactive/ReactiveKafkaConsumerTemplate.java

@@ -66,7 +66,7 @@ public Flux<ReceiverRecord<K, V>> receive() {
 	}
 
 	public Flux<ConsumerRecord<K, V>> receiveAutoAck() {
-		return this.kafkaReceiver.receiveAutoAck().flatMap(Function.identity());
+		return this.kafkaReceiver.receiveAutoAck().concatMap(Function.identity());
 	}
 
 	public Flux<ConsumerRecord<K, V>> receiveAtMostOnce() {