refactor: align S3 multi-part size to fetch part

To optimize latency on fetching, AWS recommends for upload part to be the same as ranges to download: https://docs.aws.amazon.com/whitepapers/latest/s3-optimizing-performance-best-practices/use-byte-range-fetches.html

storage/s3/src/main/java/io/aiven/kafka/tieredstorage/storage/s3/S3StorageConfig.java

@@ -65,7 +65,7 @@ public class S3StorageConfig extends AbstractConfig {
         + "Valid values: between 5MiB and 2GiB";
     static final int S3_MULTIPART_UPLOAD_PART_SIZE_MIN = 5 * 1024 * 1024; // 5MiB
     static final int S3_MULTIPART_UPLOAD_PART_SIZE_MAX = Integer.MAX_VALUE;
-    static final int S3_MULTIPART_UPLOAD_PART_SIZE_DEFAULT = S3_MULTIPART_UPLOAD_PART_SIZE_MIN;
+    static final int S3_MULTIPART_UPLOAD_PART_SIZE_DEFAULT = 16 * 1024 * 1024; // 16MiB
 
     private static final String S3_API_CALL_TIMEOUT_CONFIG = "s3.api.call.timeout";
     private static final String S3_API_CALL_TIMEOUT_DOC = "AWS S3 API call timeout in milliseconds";