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/Vcs/Reference.php

@@ -0,0 +1,51 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p7\Vcs;
+
+if ( !class_exists(Reference::class, false) ):
+
+	/**
+	 * This class represents a VCS branch or tag. It's intended as a read only, short-lived container
+	 * that only exists to provide a limited degree of type checking.
+	 *
+	 * @property string $name
+	 * @property string|null version
+	 * @property string $downloadUrl
+	 * @property string $updated
+	 *
+	 * @property string|null $changelog
+	 * @property int|null $downloadCount
+	 */
+	class Reference {
+		private $properties = array();
+
+		public function __construct($properties = array()) {
+			$this->properties = $properties;
+		}
+
+		/**
+		 * @param string $name
+		 * @return mixed|null
+		 */
+		public function __get($name) {
+			return array_key_exists($name, $this->properties) ? $this->properties[$name] : null;
+		}
+
+		/**
+		 * @param string $name
+		 * @param mixed $value
+		 */
+		public function __set($name, $value) {
+			$this->properties[$name] = $value;
+		}
+
+		/**
+		 * @param string $name
+		 * @return bool
+		 */
+		public function __isset($name) {
+			return isset($this->properties[$name]);
+		}
+
+	}
+
+endif;