### 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
CreateBucketOutput,
    CreateBucketRequest,
    CreateMultipartUploadOutput,
    CreateMultipartUploadRequest,
    Delete,
    DeleteObjectOutput,
    DeleteObjectRequest,
    DeleteObjectTaggingOutput,
    DeleteObjectTaggingRequest,
    DeleteResult,
    ETag,
    GetBucketAclOutput,
    GetBucketCorsOutput,
    GetBucketLifecycleConfigurationOutput,
    GetBucketLifecycleOutput,
    GetBucketLocationOutput,
    GetBucketReplicationOutput,
    GetBucketRequestPaymentOutput,
    GetBucketRequestPaymentRequest,
    GetBucketWebsiteOutput,
    GetObjectAclOutput,
    GetObjectAttributesOutput,
    GetObjectAttributesParts,
    GetObjectAttributesRequest,
    GetObjectOutput,
    GetObjectRequest,
    GetObjectTaggingOutput,
    GetObjectTaggingRequest,
    HeadObjectOutput,
    HeadObjectRequest,