### 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
    ) -> HttpResponse:
        """
        Serializes a given response dict (the return payload of a service implementation) to an _event stream_ using the
        given operation model.
        :param response: dictionary containing the payload for the response
        :param operation_model: describing the operation the response dict is being returned by
        :param mime_type: Mime type which should be used to encode the payload
        :return: HttpResponse which can directly be sent to the client (in chunks)
        """
        event_stream_shape = operation_model.get_event_stream_output()
        event_stream_member_name = operation_model.output_shape.event_stream_name
        # wrap the generator in operation specific serialization
        def event_stream_serializer() -> Iterable[bytes]:
            yield _event_payload("initial-response")
            # yield convert_to_binary_event_payload("", event_type="initial-response")
            # create a default response
            serialized_event_response = self._create_default_roperation_model, mime_type)
            # get the members of the event stream shape
            event_stream_shape_members = (
                event_stream_shape.members if event_stream_shape is not None else None
            )
            # extract the generator from the given response data
            event_generator = response.get(event_stream_member_name)
            if not isinstance(event_generator, Iterator):
                raise ProtocolSerializerError(
                    "Expected iterator for streaming event serialization."
                )
            # yield one event per generated event
            for event in event_generator:
                # find the actual event payload (the member with event=true)
                event_member_shape = None
                event_member_name = None
                for member_name, member_shape ers.items():
                    if member_shape.serialization.get("event") and membe