### 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
r_name in event:
                        shape
                        event_member_name = member_name
                        break
                if event_member_shape is None:
                     shape for serialization.")
                # serialize the part of the response for the event
                self._serialize_r
                    event.get(event_member_name),
                    serialized_event_response,
                    event_member_shape,
                    rs if event_member_shape is not None else None,
                    operation_model,
                    mime_type,
                )
                # execute additional response traits (might be modifying the response)
                serialized_event_response = self._prepare_additional_traits_in_r
                    serialized_event_response, operation_model
                )
                # encode the event and yield it
                yield _event_payload(
                    event_type=event_member_name, content=serialized_event_response.data
                )
        return HttpR
            response=event_stream_serializer(),
            , 200),
        )
    def _encode_event_payload(
        self,
        event_type: str,
        content: Union[str, bytes] = "",
        error_code: Optional[str] = None,
        error_message: Optional[str] = None,
    ) -> bytes:
        """
        Encodes the given event payload according to AWS specific binary event encoding.
        A specification of the format can be found in the AWS docs:
        https://docs.aws.amazon.com/AmazonS3/latest/API/RESTSelectObjectAppendix.html
        :param content: string or bytes of the event payload
        :param event_type: type of the event. Usually the name of the event shape 
                            "initial-response".
        :param error_code: Optional. Error code if the payload represents an error.
        :param error_message: Optional. Error message if the payload represents an error.
    