Accessibility: Improve the semantics of "tabs" that are not real ARIA tabs.

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.

Built from https://develop.svn.wordpress.org/trunk@44905


git-svn-id: http://core.svn.wordpress.org/trunk@44736 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/about-rtl.css

@@ -401,11 +401,13 @@
 
 /* Credits */
 
-.about-wrap h3.wp-people-group {
+.about-wrap h2.wp-people-group {
 	margin: 2.6em 0 1.33em;
 	padding: 0;
 	font-size: 16px;
 	line-height: inherit;
+	font-weight: 600;
+	text-align: right;
 }
 
 .about-wrap .wp-people-group {