Describe the bug
----------------

Adding the fullscreen prop to Modal isn't making the Modal go fullscreen.

Reproducible Example
--------------------

Minimal example by using [CodeSandbox](https://codesandbox.io/s/dreamy-panka-9ep6b?file=/src/App.js&resolutionWidth=320&resolutionHeight=675).

Expected behavior
-----------------

The modal to be fullscreen.

Screenshots
-----------

This is the example on Code Sandbox. It's also not working on my app. In my app I'm using the latest version of the react-bootstrap package. The Code Sandbox example is using 1.5.2.
![Screen Shot 2021-09-22 at 10 20 28 AM](https://user-images.githubusercontent.com/53917177/134373075-ea7692f5-5bf8-48cc-81a9-8e086d6d37dd.png)

Environment
-----------

* Operating System: macOS M1
* Browser, Version: Chrome v94
* React-Bootstrap Version: 1.5.2 on Code Sandbox. 1.6.3 in my app.

Additional context
------------------

I'm confused because the fullscreen prop seems to be working in the example in the docs. You can run a phone emulator in Chrome [here](https://react-bootstrap.github.io/components/modal/#modal-fullscreen) and see it working.
I don't see any important difference between the code in my app and the docs. Am I missing something? Doing something wrong?
Lastly, I am trying to affect only one particular modal in my app, and I only want the modal to go fullscreen when the screen size is small.
Any help is greatly appreciated!