[accessibility] Consider replacing aria-pressed with aria-expanded on the attribution control

[Malvoz]

Regarding:

mapbox-gl-js/src/ui/control/attribution_control.js

Lines 103 to 111 in 36533f3

	_toggleAttribution() {
	if (this._container.classList.contains('mapboxgl-compact-show')) {
	this._container.classList.remove('mapboxgl-compact-show');
	this._compactButton.setAttribute('aria-pressed', 'false');
	} else {
	this._container.classList.add('mapboxgl-compact-show');
	this._compactButton.setAttribute('aria-pressed', 'true');
	}
	}

aria-pressed is similar to aria-expanded in that it indicates the state of the button, but aria-expanded is more appropriate as it specifically:

Indicates whether the element, or another grouping element it controls, is currently expanded or collapsed.

The distinction is that with aria-expanded the (non-visual) user will expect other ("new") content to be available after pressing the button. This is mostly important as there is no focus delegation (which is fine if it's conveyed to the user that new content is available through aria-expanded).