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 -- redirect_upgradepage_to_location

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

Author: printminion-co

lib/base.php

@@ -360,6 +360,15 @@ private static function printUpgradePage(\OC\SystemConfig $systemConfig): void {
 			$tmpl->assign('productName', 'Nextcloud');
 		}
 		$tmpl->assign('oldTheme', $oldTheme);
+
+		$redirectUpgradePage = $systemConfig->getValue('redirect_upgradepage_to_location', null);
+		if ($redirectUpgradePage !== null) {
+			http_response_code(307);
+			header('X-Nextcloud-needsDbUpgrade: 1');
+			header("Location: ${redirectUpgradePage}#" . time());
+			die();
+		}
+
 		$tmpl->printPage();
 	}