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/class-wp-network.php

@@ -306,7 +306,7 @@ class WP_Network {
 		}
 
 		$this->cookie_domain = $this->domain;
-		if ( 'www.' === substr( $this->cookie_domain, 0, 4 ) ) {
+		if ( str_starts_with( $this->cookie_domain, 'www.' ) ) {
 			$this->cookie_domain = substr( $this->cookie_domain, 4 );
 		}
 	}