Remove deprecated Acknowledgment.nack(long) method

nack(Duration) should be used instead.

spring-kafka/src/main/java/org/springframework/kafka/support/Acknowledgment.java

@@ -36,21 +36,6 @@ public interface Acknowledgment {
 	 */
 	void acknowledge();
 
-	/**
-	 * Negatively acknowledge the current record - discard remaining records from the poll
-	 * and re-seek all partitions so that this record will be redelivered after the sleep
-	 * time (in milliseconds). Must be called on the consumer thread.
-	 * <p>
-	 * @param sleepMillis the time to sleep in milliseconds; the actual sleep time will be larger
-	 * of this value and the container's {@code maxPollInterval}, which defaults to 5 seconds.
-	 * @since 2.3
-	 * @deprecated in favor of {@link #nack(Duration)}
-	 */
-	@Deprecated(since = "2.8.7", forRemoval = true)
-	default void nack(long sleepMillis) {
-		nack(Duration.ofMillis(sleepMillis));
-	}
-
 	/**
 	 * Negatively acknowledge the current record - discard remaining records from the poll
 	 * and re-seek all partitions so that this record will be redelivered after the sleep