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/post.php

@@ -6762,7 +6762,7 @@ function wp_mime_type_icon( $mime = 0 ) {
 				if ( $dh ) {
 					while ( false !== $file = readdir( $dh ) ) {
 						$file = wp_basename( $file );
-						if ( '.' === substr( $file, 0, 1 ) ) {
+						if ( str_starts_with( $file, '.' ) ) {
 							continue;
 						}
 
@@ -7813,7 +7813,7 @@ function wp_add_trashed_suffix_to_post_name_for_post( $post ) {
 
 	$post = get_post( $post );
 
-	if ( '__trashed' === substr( $post->post_name, -9 ) ) {
+	if ( str_ends_with( $post->post_name, '__trashed' ) ) {
 		return $post->post_name;
 	}
 	add_post_meta( $post->ID, '_wp_desired_post_slug', $post->post_name );