Some "visual tabs" in the WordPress admin used links wrapped in a `h2` heading. While these tabs look like ARIA tabs, they're just navigation menus and should be marked-up as such to be correctly exposed to assistive technologies.
- changes the wrapping `<h2>` to a `<nav>` element: worth reminding `<nav>` elements also define ARIA landmarks
- adds an `aria-label` to the `<nav>` elements so they can be distinguished from other `<nav>` elements in the page
- adjusts the headings level in the Credits page
Props audrasjb, afercia.
Fixes#43398.
git-svn-id: https://develop.svn.wordpress.org/trunk@44905 602fd350-edb4-49c9-b593-d223f7449a82
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