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/v5p6/Update.php

@@ -0,0 +1,38 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p6;
+
+use stdClass;
+
+if ( !class_exists(Update::class, false) ):
+
+	/**
+	 * A simple container class for holding information about an available update.
+	 *
+	 * @author Janis Elsts
+	 * @access public
+	 */
+	abstract class Update extends Metadata {
+		public $slug;
+		public $version;
+		public $download_url;
+		public $translations = array();
+
+		/**
+		 * @return string[]
+		 */
+		protected function getFieldNames() {
+			return array('slug', 'version', 'download_url', 'translations');
+		}
+
+		public function toWpFormat() {
+			$update = new stdClass();
+
+			$update->slug = $this->slug;
+			$update->new_version = $this->version;
+			$update->package = $this->download_url;
+
+			return $update;
+		}
+	}
+
+endif;