### 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
"""Change casing of Last-Modified and other headers to be picked by the parser"""
        status_code, resp_headers, key_value = fn(self, *args, **kwargs)
        for low_case_header in [
            "last-modified",
            "content-type",
            "content-length",
            "content-range",
            "content-encoding",
            "content-language",
            "content-disposition",
            "cache-control",
        ]:
            if header_value := resp_headers.pop(low_case_header, None):
                header_name = capitalize_header_name_from_snake_case(low_case_header)
                resp_headers[header_name] = header_value
        
        
        bucket_key_enabled = "x-amz-server-side-encryption-bucket-key-enabled"
        if val := resp_headers.get(bucket_key_enabled, ""):
            resp_headers[bucket_key_enabled] = str(val).lower()
        return status_code, resp_headers, key_value
    @patch(moto_s3_responses.S3Response._bucket_response_head)
    def _fix_bucket_response_head(fn, self, bucket_name, *args, **kwargs):
        code, headers, body = fn(self, bucket_name, *args, **kwargs)
        bucket = self.backend.get_bucket(bucket_name)
        headers["x-amz-bucket-region"] = bucket.region_name
        headers["content-type"] = "application/xml"
        return code, headers, body
    @patch(moto_s3_responses.S3Response._key_response_get)
    def _fix_key_response_get(fn, *args, **kwargs):