### 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
n the operation model (if defined), otherwise 200.
        :param operation_model: ault HTTP status code
        :param mime_type: Mime type which should be used to encode the payload
        :return: boiTP response
        """
        return HttpR, 200))
    def _get_ headers: Optional[Dict | Headers]) -> str:
        """
        Extracts the accepted mom the request headers and returns a matchted mime type for the
        seriafault mime type of the service if there is no match.
        :param headers: to extract the "Accept" header from
        :return: preferred mime type to be used by the serializer (if it is not accepted by the client,
                 an error is logged)
        """
        accept_header = None
        if headers and "Accept" in headers and not headers.get("Accept") == "*/*":
            accept_header = headers.get("Accept")
        elif headers and headers.get("Content-Type"):
            # If there is no specific Accept header given, we use the given Content-Type as a fallback.
            # i.e. if the request content was JSON encoded and the client doesn't send a specific an Accept header, the
            # serializer should prefer JSON encoding.
            content_type = headers.get("Content-Type")
            LOG.debug(
                "No accept header given. Using request's Content-Type (%s) as preferred response Content-Type.",
                content_type,
            )
            accept_header = content_type + ", */*"
        mime_accept: MIMEAccept = parse_accept_header(accept_header, MIMEAccept)
        mime_type = mime_accept.best_match(self.SUPPORTED_MIME_TYPES)
        if not mime_type:
            # There is no match between the supported mime types and the requested one(s)
            mime_type = self.SUPPORTED_MIME_TYPES[0]
            LOG.debug(
                "Determined accept type (%s) is not supported by this serializer. Using default of this serializer: %s",
                accept_header,
                mime_type,