fix: Fix SQL error for MySQL vs PGSQL in Challenges delete

lib/databaseCommand/ChallengesDbCmd.php

@@ -4,6 +4,8 @@ declare(strict_types=1);
 
 namespace SimpleSAML\Module\perun\databaseCommand;
 
+use SimpleSAML\Module\perun\Exception;
+
 class ChallengesDbCmd extends DatabaseCommand
 {
     private const CHALLENGES_TABLE_NAME = 'scriptChallenges';
@@ -64,8 +66,15 @@ class ChallengesDbCmd extends DatabaseCommand
 
     public function deleteOldChallenges(): bool
     {
+        if ($this->isMysql()) {
+            $filter = ' < (NOW() - INTERVAL 5 MINUTE)';
+        } elseif ($this->isPgsql()) {
+            $filter = ' < (NOW() - INTERVAL \'5 MINUTES\')';
+        } else {
+            throw new \Exception("Unrecognized driver for database");
+        }
         $query = 'DELETE FROM ' . self::CHALLENGES_TABLE_NAME . ' WHERE '
-            . self::DATE_COLUMN . ' < (NOW() - INTERVAL 5 MINUTE)';
+            . self::DATE_COLUMN . $filter;
 
         $params = [];