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/bucket-notification-handlers_test.go

@@ -185,7 +185,7 @@ func testGetBucketNotificationHandler(obj ObjectLayer, instanceType, bucketName
 	filterRules := []filterRule{
 		{
 			Name:  "prefix",
-			Value: globalMinioDefaultOwnerID,
+			Value: "minio",
 		},
 		{
 			Name:  "suffix",