Code Modernization: Replace usage of strpos() with str_starts_with().

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

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

This commit replaces `0 === strpos( ... )` with `str_starts_with()` in core files, making the code more readable and consistent, as well as improving performance.

While `strpos()` is slightly faster than the polyfill on PHP < 8.0, `str_starts_with()` is noticeably faster on PHP 8.0+, as it is optimized to avoid unnecessarily searching along the whole haystack if it does not find the needle.

Follow-up to [52039], [52040], [52326].

Props spacedmonkey, costdev, sabernhardt, mukesh27, desrosj, jorbin, TobiasBg, ayeshrajans, lgadzhev, SergeyBiryukov.
Fixes #58012.
Built from https://develop.svn.wordpress.org/trunk@55703


git-svn-id: http://core.svn.wordpress.org/trunk@55215 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/plugin.php

@@ -770,7 +770,7 @@ function plugin_basename( $file ) {
 	arsort( $wp_plugin_paths );
 
 	foreach ( $wp_plugin_paths as $dir => $realdir ) {
-		if ( strpos( $file, $realdir ) === 0 ) {
+		if ( str_starts_with( $file, $realdir ) ) {
 			$file = $dir . substr( $file, strlen( $realdir ) );
 		}
 	}