Code Modernization: Use str_contains() in get_avatar_data().

`str_contains()` was introduced in PHP 8.0 to perform a case-sensitive check indicating if the string to search in (haystack) contains the given substring (needle).

WordPress core includes a polyfill for `str_contains()` on PHP < 8.0 as of WordPress 5.9.

Follow-up to [52039], [55988], [56245], [60897].

Props TobiasBg.
See #63087.

git-svn-id: https://develop.svn.wordpress.org/trunk@60900 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/link-template.php

@@ -4577,7 +4577,7 @@ function get_avatar_data( $id_or_email, $args = null ) {
 		}
 
 		if ( '' !== $name ) {
-			if ( false === strpos( $name, ' ' ) || preg_match( '/\p{Han}|\p{Hiragana}|\p{Katakana}|\p{Hangul}/u', $name ) ) {
+			if ( ! str_contains( $name, ' ' ) || preg_match( '/\p{Han}|\p{Hiragana}|\p{Katakana}|\p{Hangul}/u', $name ) ) {
 				$initials = mb_substr( $name, 0, min( 2, mb_strlen( $name, 'UTF-8' ) ), 'UTF-8' );
 			} else {
 				$first    = mb_substr( $name, 0, 1, 'UTF-8' );