Use empty string to set boolean attribute

This is a minimal way of setting the `hidden` attribute.

From MDN:

> The hidden attribute is used to indicate that the content of an element should not be presented to the user. This attribute can take any one of the following values:
>
> - an empty string
> - the keyword hidden
> - the keyword until-found
>
> An empty string, or the keyword hidden, set the element to the hidden state. Additionally, invalid values set the element to the hidden state.

https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/hidden

packages/a11y/src/script/add-intro-text.ts

@@ -30,7 +30,7 @@ export default function addIntroText() {
 			'border:0;' +
 			'word-wrap:normal !important;'
 	);
-	introText.setAttribute( 'hidden', 'hidden' );
+	introText.setAttribute( 'hidden', '' );
 
 	const { body } = document;
 	if ( body ) {