GH-2184: Don't Log KafkaBackoffException

Resolves #2184

`KafkaBackOffException`s are incorrectly logged at ERROR level during
retries when using the no-seek error handling mode.

They are not logged in the seek mode; see
448871a

**cherry-pick to 2.9.x**

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/listener/DefaultErrorHandler.java

@@ -157,7 +157,12 @@ public boolean handleOne(Exception thrownException, ConsumerRecord<?, ?> record,
 			return getFailureTracker().recovered(record, thrownException, container, consumer);
 		}
 		catch (Exception ex) {
-			logger.error(ex, "Failed to handle " + KafkaUtils.format(record) + " with " + thrownException);
+			if (SeekUtils.isBackoffException(thrownException)) {
+				this.logger.debug(ex, "Failed to handle " + KafkaUtils.format(record) + " with " + thrownException);
+			}
+			else {
+				this.logger.error(ex, "Failed to handle " + KafkaUtils.format(record) + " with " + thrownException);
+			}
 			return false;
 		}
 	}