Fix legacy update notifications

[mrtnmtth]

This fixes issue #21266

Since this is legacy code, I only changed as much as needed to fix the issue

[nickvergessen]

To test on master (where no update can be available), apply the following patch:

diff --git a/apps/updatenotification/lib/AppInfo/Application.php b/apps/updatenotification/lib/AppInfo/Application.php
index 54d6444236..4775c6ece1 100644
--- a/apps/updatenotification/lib/AppInfo/Application.php
+++ b/apps/updatenotification/lib/AppInfo/Application.php
@@ -62,6 +62,9 @@ class Application extends App implements IBootstrap {
                                                                         IGroupManager $groupManager,
                                                                         IAppContainer $appContainer,
                                                                         ILogger $logger) {
+                       Util::addScript('updatenotification', 'legacy-notification');
+                       $updateChecker = $appContainer->get(UpdateChecker::class);
+                       \OC_Hook::connect('\OCP\Config', 'js', $updateChecker, 'populateJavaScriptVariables');
                        if ($config->getSystemValue('updatechecker', true) !== true) {
                                // Updater check is disabled
                                return;
diff --git a/apps/updatenotification/lib/UpdateChecker.php b/apps/updatenotification/lib/UpdateChecker.php
index b83eff8818..7bcc29a4fd 100644
--- a/apps/updatenotification/lib/UpdateChecker.php
+++ b/apps/updatenotification/lib/UpdateChecker.php
@@ -75,7 +75,15 @@ class UpdateChecker {
                        return $result;
                }
 
-               return [];
+               return [
+                       'updateAvailable' => true,
+                       'updateVersion' => '20.0.0',
+                       'updateVersionString' => '20.0.0',
+                       'updaterEnabled' => true,
+                       'versionIsEol' => false,
+                       'updateLink' => 'https://nextcloud.com',
+                       'downloadLink' => 'https://nextcloud.com',
+               ];
        }
 
        /**
@@ -84,8 +92,8 @@ class UpdateChecker {
        public function populateJavaScriptVariables(array $data) {
                $data['array']['oc_updateState'] =  json_encode([
                        'updateAvailable' => true,
-                       'updateVersion' => $this->getUpdateState()['updateVersionString'],
-                       'updateLink' => $this->getUpdateState()['updateLink'] ?? '',
+                       'updateVersion' => '20.0.0',// $this->getUpdateState()['updateVersionString'],
+                       'updateLink' => 'https://nextcloud.com',//  $this->getUpdateState()['updateLink'] ?? '',
                ]);
        }
 }

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[nickvergessen]

/backport to stable20

[nickvergessen]

/backport to stable19