a11y: Do not mark top bar notifications area as aria-hidden

Summary:
Do not the mark main alerts/notifications area as aria-hidden.
Focusable descendents within an [aria-hidden="true"] element prevent those interactive elements from being available to users of assistive technologies like screen readers.

As can be seen in the code of `PhabricatorMainMenuView`, all three icons already define some `$aural[]` array to include output for assistive technologies, so their parent element should not block accessing that output.

Ref T16072

Test Plan:
* Go to http://phorge.localhost/ and inspect the `<div class="phabricator-main-menu-alerts">` element and its descendents.
* Ideally, check with some screenreader sofware. I didn't.
* Optionally, run an Accessibility check in Chromium Lighthouse.

Reviewers: O1 Blessed Committers, chris

Reviewed By: O1 Blessed Committers, chris

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16072

Differential Revision: https://we.phorge.it/D26053