### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

When requesting a  Complete operation at the end of a multi part upload, the parts have to be sorted in ascending order of part number.
AWS S3 issues an error if it is not the case. localstack does not generate such error (although, its log issues warnings)/

### Expected Behavior

Complete should generate an error

### How are you starting LocalStack?

With the `localstack` script

### Steps To Reproduce

localstack start

Buggy software extract (simplified)
```golang
	wg := errgroup.Group{}
	for i := 0; i < 3; i++ {
		wg.Go(func() error {
			rd, err := os.Open(source)
			if err != nil {
				return err
			}
			return u.Upload(rd, i, 10*_cMB)  // uploads and stores for part i
		})
	}
	err :=  wg.Wait()
if err != nil {
...
}
err = u.Complete()
// as using i inside the routines, all the four parts will have the same value 3
```

### Environment

```markdown
- OS:Ubuntu 20.04
- LocalStack: latest pro
```


### Anything else?

_No response_
assert "content-type" in response.headers
        assert "text/html" == response.headers["content-type"]
        assert "etag" in response.headers
        assert actual_key_obj["ETag"] in response.headers["etag"]
        # If-N
        response = .get(
            url, headers={"If-None-Match": actual_key_obj["ETag"]}, verify=False
        )
        assert 304 == response.status_code
        # key 
        url = f"{website_url}/with-content-type/key.js"
        response = .get(url, verify=False)
        assert 200 == response.status_code
        assert "some js" == response.text
        assert "content-type" in response.headers
        assert "application/javascript; charset=utf-8" == response.headers["content-type"]
        assert "etag" in response.headers
        assert with_content_type_obj["ETag"] == response.headers["etag"]
        # inde
        url = f"{website_url}/test"
        response = .get(url, verify=False)
        assert 200 == response.status_code
        assert "index" == response.text
        assert "content-type" in response.headers
        assert "text/html" in response.headers["content-type"]
        assert "etag" in response.headers
        assert index_obj["ETag"] == response.headers["etag"]
        # root 
        url = f"{website_url}/"
        response = .get(url, verify=False)