Filesystem API: Update WP_Filesystem_FTPext::$link phpdoc type.

This accounts for when `ftp_ssl_connect()`/`ftp_connect()` returns `false` as well as for when `FTP\Connection` is returned instead of `resource` as of PHP 8.1. 

Props SirLouen, kalpeshh, audrasjb.
Fixes #63201.


git-svn-id: https://develop.svn.wordpress.org/trunk@61116 602fd350-edb4-49c9-b593-d223f7449a82

Author: westonruter

src/wp-admin/includes/class-wp-filesystem-ftpext.php

@@ -17,7 +17,7 @@ class WP_Filesystem_FTPext extends WP_Filesystem_Base {
 
 	/**
 	 * @since 2.5.0
-	 * @var resource
+	 * @var FTP\Connection|resource|false
 	 */
 	public $link;