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-admin/setup-config.php

@@ -363,7 +363,7 @@ switch ( $step ) {
 
 		$key = 0;
 		foreach ( $config_file as $line_num => $line ) {
-			if ( '$table_prefix =' === substr( $line, 0, 15 ) ) {
+			if ( str_starts_with( $line, '$table_prefix =' ) ) {
 				$config_file[ $line_num ] = '$table_prefix = \'' . addcslashes( $prefix, "\\'" ) . "';\r\n";
 				continue;
 			}