### 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
: str,
    ) -> None:
        """
        Serializes the given parameters as XML for the query protocol.
        :param parameters: The user input params
        :param response: The final serialized HttpResponse
        :param shy" response)
        :param shape_members: The members of the output struct shape
        :param operation_model: The specification of the operation of which the response is serialized here
        :param mime_type: Mime type which should be used to encode the payload
        :return: None - the given `serialized` dict is modified
        """
        response.set_r
            _payload(
                peration_model, mime_type)
            )
        )
    def _serialize_body_params_to_xml(
        self, params: dict, shape: Shape, operation_model: OperationModel, mime_type: str
    ) -> ETree.Element:
        # The Query protocol responses have a root element which is not contained in the specification file.
        # Therefore, we first call the super function to perform the normal XML serialization, and afterwards wrap the
        # result in a root element based on the operation name.
        node = super()._serialize_body_params_to_xml(params, shape, operation_model, mime_type)
        # Check if we need to add a namespace
        attr = (
            {"xmlns": operation_model.metadata.get("xmlNamespace")}
            if "xmlNamespace" in operation_model.metadata
            else None
        )
        # Create the root element and add the result of the XML serializer as a child node
        root = ETree.Element(f"{operation_model.name}Response", attr)
        if node is not None:
            root.append(node)
        return root
    ot: Optional[ETree.Element]):
        # Add the response metadata here (it's not defined in the specs)
        # For the ec2 and the query protocol, the root cannot be None at this time.
        response_metadata = ETree.SubElement(root, "ResponseMetadata")
        request_id = ETree.SubElement(response_me