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/plugin-update-checker.php

@@ -1,10 +1,10 @@
 <?php
 /**
- * Plugin Update Checker Library 5.4
+ * Plugin Update Checker Library 5.7
  * http://w-shadow.com/
  *
- * Copyright 2024 Janis Elsts
+ * Copyright 2026 Janis Elsts
  * Released under the MIT license. See license.txt for details.
  */
 
-require dirname(__FILE__) . '/load-v5p4.php';
+require dirname(__FILE__) . '/load-v5p7.php';