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.
Built from https://develop.svn.wordpress.org/trunk@55990


git-svn-id: http://core.svn.wordpress.org/trunk@55502 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/deprecated.php

@@ -1365,7 +1365,7 @@ function wp_dashboard_plugins_output( $rss, $args = array() ) {
 			// Is this random plugin's slug already installed? If so, try again.
 			reset( $plugin_slugs );
 			foreach ( $plugin_slugs as $plugin_slug ) {
-				if ( $slug === substr( $plugin_slug, 0, strlen( $slug ) ) ) {
+				if ( str_starts_with( $plugin_slug, $slug ) ) {
 					unset( $items[$item_key] );
 					continue 2;
 				}