Coding Standards: Replace include_once with require_once for required files.

Per [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#writing-include-require-statements WordPress PHP coding standards], it is ''strongly recommended'' to use `require[_once]` for unconditional includes. When using `include[_once]`, PHP will throw a warning when the file is not found but will continue execution, which will almost certainly lead to other errors/warnings/notices being thrown if your application depends on the file loaded, potentially leading to security leaks. For that reason, `require[_once]` is generally the better choice as it will throw a `Fatal Error` if the file cannot be found.

Follow-up to [1674], [1812], [1964], [6779], [8540], [10521], [11005], [11911], [16065], [16149], [25421], [25466], [25823], [37714], [42981], [45448], [47198], [54276], [55633].

Props kausaralm, SergeyBiryukov.
See #57839.

git-svn-id: https://develop.svn.wordpress.org/trunk@55641 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/cron.php

@@ -883,7 +883,7 @@ function spawn_cron( $gmt_time = 0 ) {
 		wp_ob_end_flush_all();
 		flush();
 
-		include_once ABSPATH . 'wp-cron.php';
+		require_once ABSPATH . 'wp-cron.php';
 		return true;
 	}