fix: OwnerID in response should be 64 character in length. (#4554)

Rather than sending a custom "minio" string, we can
change this to `sha256('arn:aws:iam::minio:user/admin')`.

Fixes #4553

cmd/object-api-multipart_test.go

@@ -1061,7 +1061,7 @@ func testListMultipartUploads(obj ObjectLayer, instanceType string, t TestErrHan
 		{
 			MaxUploads:     10,
 			IsTruncated:    false,
-			Prefix:         globalMinioDefaultOwnerID,
+			Prefix:         "minio",
 			UploadIDMarker: uploadIDs[4],
 			Uploads: []uploadMetadata{
 				{