[Bug] Duplicate iframes created when <CldUploadWidget /> re-renders

# Bug Report

## Describe the bug

The bug occurs when using conditional rendering: `` can create multiple iframes instead of just one.

## Is this a regression?

I'm not sure. I will check if this issue existed in previous versions and update the report accordingly."

## Steps To Reproduce the error

1. Create a new empty React/Next.js project.
2. Install next-cloudinary@^6.16.0.
3. Render the `` component.
4. Create a state variable with an initial value of false.
5. Create a `useEffect()` hook where:
- The state variable is set to true at the beginning.
- An asynchronous task is simulated using `setTimeout()` or any other method.
- Once the task completes successfully, the state is set back to false.
7. Implement conditional rendering before rendering the `` component. This can be a text message, a loader, or any other UI component.

## Expected behaviour

I expect that when `` is unmounted and then re-mounted, there will be only one iframe inside the DOM tree.

## CodeSandbox or Live Example of Bug

[Live CodeSandbox bug example](https://codesandbox.io/p/devbox/cld-upload-widget-duplicating-iframe-9cx7td)

Don't forget to create .env.local with speicified variables:
```
NEXT_PUBLIC_CLOUDINARY_CLOUD_NAME=
NEXT_PUBLIC_CLOUDINARY_API_KEY=
CLOUDINARY_API_SECRET=
```

## Screenshot or Video Recording

[Example of two iframes](https://github.com/user-attachments/assets/31c5e780-8aa5-4994-b0c8-f5406204425b)

## Your environment

- OS: Arch Linux x86_64
- Node version: v22.13.1
- Npm version: 10.9.2
- Browser name and version: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/131.0.0.0 Safari/537.36

## Additional context

This behavior is quite non-obvious. I personally discovered this issue while writing E2E tests, as my test started failing due to the presence of two iframes instead of one.

It took me a significant amount of time to analyze and understand the root cause of this behavior.