Add aria-label attribute to navigation menu blocks

Problem/Motivation

This is follow up issue from https://www.drupal.org/project/drupal/issues/3393400#comment-15627607

As part of that issue, the aria-labelled-by attributes were removed from the HTML markup. (Commit)

In order to solve the accessibility issues introduced by that change, an alternative has been proposed by the accessibility team:

<nav aria-label="Title for navigation">
  <ul>
    <li><a href="page11.html">Link 1</a></li>
    <li><a href="page12.html">Link 2</a></li>
    .....
  </ul>
</nav>

Necessary changes in menu blocks markup need to be done to adapt them to achieve the requested outcome.

Proposed resolution

• Introduce a <nav aria-label="Title for navigation"> wrapper for each menu used with Navigation, as noted above.
• Introducing that will make the existing wrapping <nav id="menu-builder"> and <nav id="menu-footer"> elements redundant. Lets remove those.
• Finally, while we're reviewing the labels for menus within the Navigation, lets add an aria-label to the Navigation side bar <aside> element, which currently just reads as "complementary". Add label "Administrative side bar". This aligns nicely with "Administrative top bar" label that we already have for the <aside> element that wraps the Top Bar.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

None