### 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
_item is not None:
                wrapper = {}
                # The JSON list serialization is the only case where we aren't
                # setting a key on a dict.  We handle this by using
                # a __current__ key on a wrapper dict to serialize each
                # list item before appending it to the serialized list.
                self._serialize(wrapper, list_item, shape.member, "__current__", mime_type)
                list_obj.append(wrapper["__current__"])
    def _default_serialize(self, body: dict, value: Any, _, key: str, __):
        body[key] = value
    def _serialize_type_timestamp(
        self, body: dict, value: Any, shape: Shape, key: str, mime_type: str
    ):
        timestamp_format = (
            shape.serialization.get("timestampFormat")
            # CBOR always uses unix timestamp milliseconds
            if mime_type not in self.CBOR_TYPES
            else "unixtimestampmillis"
        )
        body[key] = self._convert_timestamp_to_str(value, timestamp_format)
    def _serialize_type_blob(
        self, body: dict, value: Union[str, bytes], _, key: str, mime_type: str
    ):
        if mime_type in self.CBOR_TYPES:
            body[key] = value
        else:
            body[key] = self._get_base64(value)
    def _prepare_additional_traits_in_r
        self, response: HttpResponse, operation_model: OperationModel
    ):
        response.headers["x-amzn-requestid"] = gen_amzn_requestid_long()
        response = super()._prepare_additional_traits_in_rresponse, operation_model)
        return response
class RestJSONResponseSerializer(BaseRestResponseSerializer, JSONResponseSerializer):
    """
    The ``RestJSONResponseSerializer`` is responsible for the serialization of responses from services with the
    ``rest-json`` protocol.
    It combines the ``BaseRestResponseSerializer`` (for the ReST specific logic) with the ``JSONResponseSerializer``
    (for the JSOn body response serialization).
    """
    def _serialize_co