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/languages/plugin-update-checker-sr_RS.po

@@ -0,0 +1,57 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: plugin-update-checker\n"
+"POT-Creation-Date: 2024-12-09 11:45+0100\n"
+"PO-Revision-Date: 2024-12-09 12:20+0100\n"
+"Last-Translator: Aleksandar Urošević <urke.kg@gmail.com>\n"
+"Language-Team: \n"
+"Language: sr_RS\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"Plural-Forms: nplurals=2; plural=(n != 1);\n"
+"X-Generator: Poedit 2.4.3\n"
+"X-Poedit-Basepath: ..\n"
+"X-Poedit-SourceCharset: UTF-8\n"
+"X-Poedit-KeywordsList: __;_e;_x:1,2c;_x\n"
+"X-Poedit-SearchPath-0: .\n"
+
+#: Puc/v5p5/Plugin/Ui.php:56
+msgid "View details"
+msgstr "Види детаље"
+
+#: Puc/v5p5/Plugin/Ui.php:79
+#, php-format
+msgid "More information about %s"
+msgstr "Више информација о %s"
+
+#: Puc/v5p5/Plugin/Ui.php:130
+msgid "Check for updates"
+msgstr "Провера ажурирања"
+
+#: Puc/v5p5/Plugin/Ui.php:217
+#, php-format
+msgctxt "the plugin title"
+msgid "The %s plugin is up to date."
+msgstr "Додатак %s је у најновијем издању."
+
+#: Puc/v5p5/Plugin/Ui.php:219
+#, php-format
+msgctxt "the plugin title"
+msgid "A new version of the %s plugin is available."
+msgstr "Доступно је ново издање за %s."
+
+#: Puc/v5p5/Plugin/Ui.php:221
+#, php-format
+msgctxt "the plugin title"
+msgid "Could not determine if updates are available for %s."
+msgstr "Није могуће утврдити да ли су доступне исправке за %s."
+
+#: Puc/v5p5/Plugin/Ui.php:227
+#, php-format
+msgid "Unknown update checker status \"%s\""
+msgstr "Непознат статус провере ажурирања \"%s\""
+
+#: Puc/v5p5/Vcs/PluginUpdateChecker.php:113
+msgid "There is no changelog available."
+msgstr "Белешке о изменама нису доступне."