### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Initiating a multipart S3 upload to LocalStack returns an XML document with a root element CreateMultipartUploadOutput. This causes an error in the ex_aws_s3 library I'm using. We've used the combination of LocalStack + ex_aws_s3 successfully for a few years and the error started occurring a few days ago.

### Expected Behavior

The element should be called InitiateMultipartUploadResult.

### How are you starting LocalStack?

With a `docker run` command

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run -p 4566:4566 localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    aws --debug --endpoint-url=http://localhost:4566 s3api create-multipart-upload --bucket my-bucket --key 'multipart/01'


### Environment

```markdown
- OS: Ubuntu 20.04.5 LTS
- LocalStack: latest
```


### Anything else?

S3 docs specifying InitiateMultipartUploadResult:
https://docs.aws.amazon.com/AmazonS3/latest/API/API_CreateMultipartUpload.html

Reference in official AWS Java client:
https://github.com/aws/aws-sdk-java/blob/ef22b94750d4c155d74b09012b3be5d9d98bbd33/aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/model/transform/XmlResponsesSaxParser.java#L1865

Where our particular error gets raised in ex_aws_s3:
https://github.com/ex-aws/ex_aws_s3/blob/7e12f2b0578b2620c047d2e44c40a6de8773584a/lib/ex_aws/s3/parsers.ex#L78
erialization
        serialized_response = self._create_default_roperation_model, mime_type)
        if not error or not isinstance(error, ServiceException):
            raise ProtocolSerializerError(
                f"Error to serialize ({error.__class__.__name__ if error else None}) is not a ServiceException."
            )
        shape = operation_model.service_model.shape_for_error_code(error.code)
        serialized_response.status_code = error.status_code
        self._serialize_error(error, serialized_response, shape, operation_model, mime_type)
        serialized_response = self._prepare_additional_traits_in_r
            serialized_response, operation_model
        )
        return serialized_response
    def _serialize_r
        self,
        parameters: dict,
        response: HttpResponse,
        shape: Optional[Shape],
        shape_members: dict,
        operation_model: OperationModel,
        mime_type: str,
    ) -> None:
        raise NotImplementedError
    def _serialize_body_params(
        self,
        params: dict,
        shape: Shape,
        operation_model: OperationModel,
        mime_type: str,
    ) -> Optional[str]:
        """
        Actually serializes the given params for the given shape to a string for the transmission in the body of the
        response.
        :param params: to serialize
        :param shape: to know how to serialize the params
        :param operation_model: for additional metadata
        :param mime_type: Mime type which should be used to encode the payload
        :return: string containing the serialized body
        """
        raise NotImplementedError
    def _serialize_error(
        self,
        error: ServiceException,
        response: HttpResponse,
        shape: StructureShape,
        operation_model: OperationModel,
        mime_type: str,
    ) -> None:
        raise NotImplementedError
    def _serialize_event_stream(
        self, response: dict, operation_model: OperationModel, mime_type