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.

git-svn-id: https://develop.svn.wordpress.org/trunk@55703 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-manager.php

@@ -1981,7 +1981,7 @@ final class WP_Customize_Manager {
 				&&
 				$parsed_allowed_url['host'] === $parsed_original_url['host']
 				&&
-				0 === strpos( $parsed_original_url['path'], $parsed_allowed_url['path'] )
+				str_starts_with( $parsed_original_url['path'], $parsed_allowed_url['path'] )
 			);
 			if ( $is_allowed ) {
 				break;