IONOS(base): add redirect for upgrade page if needed

in case you dont want to display upgrade button to users
you can redirect them to other location (e.g. dedicated maintenance page)

set desired location via
./occ config:system:set  --type string --value /maintenance -- upgrade.redirect_upgradepage_to_location

Signed-off-by: Misha M.-Kupriyanov <kupriyanov@strato.de>

Author: printminion-co

lib/base.php

@@ -257,6 +257,8 @@ public static function checkMaintenanceMode(\OC\SystemConfig $systemConfig): voi
 	private static function printUpgradePage(\OC\SystemConfig $systemConfig): void {
 		$cliUpgradeLink = $systemConfig->getValue('upgrade.cli-upgrade-link', '');
 		$disableWebUpdater = $systemConfig->getValue('upgrade.disable-web', false);
+		$redirectUpgradePage = $systemConfig->getValue('upgrade.redirect_upgradepage_to_location', null);
+
 		$tooBig = false;
 		if (!$disableWebUpdater) {
 			$apps = Server::get(\OCP\App\IAppManager::class);
@@ -292,6 +294,13 @@ private static function printUpgradePage(\OC\SystemConfig $systemConfig): void {
 		$ignoreTooBigWarning = isset($_GET['IKnowThatThisIsABigInstanceAndTheUpdateRequestCouldRunIntoATimeoutAndHowToRestoreABackup']) &&
 			$_GET['IKnowThatThisIsABigInstanceAndTheUpdateRequestCouldRunIntoATimeoutAndHowToRestoreABackup'] === 'IAmSuperSureToDoThis';
 
+		if ($redirectUpgradePage !== null) {
+			http_response_code(302);
+			header('X-Nextcloud-needsDbUpgrade: 1');
+			header("Location: ${redirectUpgradePage}#" . time());
+			die();
+		}
+
 		if ($disableWebUpdater || ($tooBig && !$ignoreTooBigWarning)) {
 			// send http status 503
 			http_response_code(503);