Fix PHP 8.2 'Creation of dynamic property' warning (#360)

Author: danielbachhuber

src/WP_CLI/CommandWithUpgrade.php

@@ -13,6 +13,7 @@
 
 abstract class CommandWithUpgrade extends \WP_CLI_Command {
 
+	protected $fetcher;
 	protected $item_type;
 	protected $obj_fields;