Update symfony to 6.3 and upgrade some classes

src/Service/DatabaseService.php

@@ -10,7 +10,7 @@ class DatabaseService
     private mysqli $conn;
 
     public function __construct(
-        private MysqliConnection $service
+        private readonly MysqliConnection $service
     )
     {
         $this->conn = $this->service->getConnection();