Code Modernization: Replace usage of substr() with str_starts_with() and str_ends_with().

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

WordPress core includes a polyfill for these functions on PHP < 8.0 as of WordPress 5.9.

This commit uses `str_starts_with()` and `str_ends_with()` in core files where appropriate:
* `$needle === substr( $string, 0, $length )`, where `$length` is the length of `$needle`, is replaced with `str_starts_with( $haystack, $needle )`.
* `$needle === substr( $string, $offset )`, where `$offset` is negative and the absolute value of `$offset` is the length of `$needle`, is replaced with `str_ends_with( $haystack, $needle )`.

This aims to make the code more readable and consistent, as well as better aligned with modern development practices.

Follow-up to [52039], [52040], [52326], [55703], [55710], [55987], [55988].

Props Soean, spacedmonkey, Clorith, ocean90, azaozz, sabernhardt, SergeyBiryukov.
Fixes #58220.

git-svn-id: https://develop.svn.wordpress.org/trunk@55990 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/load.php

@@ -835,7 +835,7 @@ function wp_get_mu_plugins() {
 		return $mu_plugins;
 	}
 	while ( ( $plugin = readdir( $dh ) ) !== false ) {
-		if ( '.php' === substr( $plugin, -4 ) ) {
+		if ( str_ends_with( $plugin, '.php' ) ) {
 			$mu_plugins[] = WPMU_PLUGIN_DIR . '/' . $plugin;
 		}
 	}