### 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
be None at this time.
        request_id = ETree.SubElement(root, "requestId")
        request_id.text = gen_amzn_requestid_long()
class JSONResponseSerializer(ResponseSerializer):
    """
    The ``JSONResponseSerializer`` is responsible for the serialization of responses from services with the ``json``
    protocol. It implements the JSON response body serialization, which is also used by the
    ``RestJSONResponseSerializer``.
    """
    JSON_TYPES = [APPLICATION_JSON, APPLICATION_AMZ_JSON_1_0, APPLICATION_AMZ_JSON_1_1]
    CBOR_TYPES = [APPLICATION_CBOR, APPLICATION_AMZ_CBOR_1_1]
    SUPPORTED_MIME_TYPES = JSON_TYPES + CBOR_TYPES
    TIMESTAMP_FORMAT = "unixtimestamp"
    def _serialize_error(
        self,
        error: ServiceException,
        response: HttpResponse,
        shape: StructureShape,
        operation_model: OperationModel,
        mime_type: str,
    ) -> None:
        body = dict()
        # TODO implement different service-specific serializer configurations
        #   - currently we set both, the `__type` member as well as the `X-Amzn-Errortype` header
        #   - the specification defines that it's either the __type field OR the header
        response.headers["X-Amzn-Errortype"] = error.code
        body["__type"] = error.code
        if shape:
            remaining_params = {}
            # TODO add a possibility to serialize simple non-modelled errors (like S3 NoSuchBucket#BucketName)
            for member in shape.members:
                if hasattr(error, member):
                    remaining_params[member] = getattr(error, member)
                # Default error message fields can sometimes have different casing in the specs
                elif member.lower() in ["code", "message"] and hasattr(error, member.lower()):
                    remaining_params[member] = getattr(error, member.lower())
            self._serialize(body, remaining_params, shape, None, mime_type)
        # Only set the message if it has not been set with t