Update bundled plugin-update-checker to v5.7 and remove broken setBranch() call

The bundled PUC was missing the setBranch() method on the main UpdateChecker
class (it's only on Vcs\PluginUpdateChecker via the VcsCheckerMethods trait).
When called with a non-VCS URL like the Gitea update URL, this caused a fatal
error at plugin activation.

Updating to PUC v5.7 (May 2026) which is the latest upstream release. The
setBranch() call is removed because PUC doesn't recognize Gitea as a VCS host;
the plugin falls back to PUC's plain JSON metadata mode, which is fine for a
plugin hosted on a self-managed repo.

includes/plugin-update-checker/Puc/v5p7/Update.php

@@ -0,0 +1,38 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p7;
+
+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;