chore(deps): vendor plugin-update-checker v5.6

Source: https://github.com/YahnisElsts/plugin-update-checker/releases/tag/v5.6
Used in 1.2.0 for self-hosted updates from Gitea (Generic JSON mode).

includes/lib/plugin-update-checker/Puc/v5/PucFactory.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace YahnisElsts\PluginUpdateChecker\v5;
+
+if ( !class_exists(PucFactory::class, false) ):
+
+	class PucFactory extends \YahnisElsts\PluginUpdateChecker\v5p6\PucFactory {
+	}
+
+endif;