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/auth-rpc-server_test.go

@@ -77,7 +77,7 @@ func TestLogin(t *testing.T) {
 		// Invalid password length
 		{
 			args: LoginRPCArgs{
-				Username: globalMinioDefaultOwnerID,
+				Username: "minio",
 				Password: "aaa",
 				Version:  Version,
 			},