FocusableIframe: React does not recognize the forwardedRef prop on a DOM element

[mmtr]

Describe the bug
When rendering a FocusableIframe, the following warning message appears in the console:

Warning: React does not recognize the forwardedRef prop on a DOM element. If you intentionally want it to appear in the DOM as a custom attribute, spell it as lowercase forwardedref instead. If you accidentally passed it from a parent component, remove it from the DOM element.

To Reproduce
Execute the code below in a new React project after installing @wordpress/components.

import React from 'react';
import ReactDOM from 'react-dom';
import { FocusableIframe } from '@wordpress/components';

import '@wordpress/components/build-style/styles.css';

const MyFocusableIframe = () => (
	<FocusableIframe
		src="/"
		onFocus={ () => console.log( 'iframe is focused' ) }
	/>
);
ReactDOM.render(
	<MyFocusableIframe />,
	document.getElementById( 'root' )
);

Expected behavior
No warning message should appear.

Desktop:

• OS: macOS High Sierra
• Browser Chrome
• Version 67

Additional context
Issue found while working on #8338 and Automattic/wp-calypso#26367

[aduth]

Fixed by #8570