### 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
    :return: bytes with the AWS-specific encoded event payload
        """
        # determine the event type (error if an error message or an error code is set)
        if error_message or error_code:
            message_type = "error"
        else:
            message_type = "event"
        # set the headers
        headers = {":event-type": event_type, ":message-type": message_type}
        if error_message:
            headers[":error-message"] = error_message
        if error_code:
            headers[":error-code"] = error_code
        # construct headers
        header_section = b""
        for key, value in headers.items():
            header_name = key.encode(self.DEFAULT_ENCODING)
            header_value = to_bytes(value)
            header_section += pack("!B", len(header_name))
            header_section += header_name
            header_section += pack("!B", self.AWS_BINARY_DATA_TYPE_STRING)
            header_section += pack("!H", len(header_value))
            header_section += header_value
        # construct body
        if isinstance(content, str):
            payload = bytes(content, self.DEFAULT_ENCODING)
        else:
            payload = content
        # calculate lengths
        headers_length = len(he
        payload_length = len(payload)
        # construct message
        # - prelude
        result = pack("!I", payload_length + headers_length + 16)
        result += pack("!I", headers_length)
        # - prelude crc
        prelude_crc 
        result += pack("!I", prelude_crc)
        # - headers
        result += header_section
        # - payload
        result += payload
        # - message crc
        payload_crc 
        result += pack("!I", payload_crc)
        return result
    def _create_default_r
        self, operation_model: OperationModel, mime_type: str
    ) -> HttpResponse:
        """
        Creates a boilerplate default response to be used by subclas points.
        Uses the default HTTP response status code defined i