Help/About: Update About page header images and performance content.

This adds the correct string for performance changes, updates the header images to not have rounded corners, and updates the release badge to use the stylized 6.9 icon.

Backport includes the fast follow to spell out abbreviation in About page text.

Reviewed by wildworks.
Merges [61257,61258] to the 6.9 branch.

Props ryelle, peterwilsoncc, westonruter, wildworks.
See #63941.


git-svn-id: https://develop.svn.wordpress.org/branches/6.9@61259 602fd350-edb4-49c9-b593-d223f7449a82

Author: peterwilsoncc

src/wp-admin/about.php

@@ -138,7 +138,7 @@
 					</svg>
 				</div>
 				<h3><?php _e( 'Performance updates' ); ?></h3>
-				<p><?php _e( 'WordPress 6.9 includes a broad set of performance updates, including a faster way of browsing and selecting styles, optimizations of the navigation link, fixes the layout shifts caused by the Video block, quicker load of the terms selector, no extra network request for retrieving feature media, and optimization in the Editor.' ); ?></p>
+				<p><?php _e( 'WordPress 6.9 includes a broad set of performance enhancements. A better <abbr>LCP</abbr> (Largest Contentful Paint) metric is achieved through improved loading of conditional and inlined stylesheets, script loading with fetchpriority support, and additional core optimizations. Editor advances include fixes for layout shifts caused by the Video block and faster loading of the terms selector.' ); ?></p>
 			</div>
 			<div class="column">
 				<div class="about__image">