Add missing @deprecated annotation

Author: sugmanue

core/sdk-core/src/main/java/software/amazon/awssdk/core/retry/backoff/FixedDelayBackoffStrategy.java

@@ -29,6 +29,7 @@
  * {@link software.amazon.awssdk.retries.api.BackoffStrategy#fixedDelay(Duration)}.
  */
 @SdkPublicApi
+@Deprecated
 public final class FixedDelayBackoffStrategy implements BackoffStrategy {
 
     private final Duration fixedBackoff;