chore (Symfony) use Symfony 5 + strict typing

Update Symfony version to 5
Use strict typing

GregwarCaptchaBundle.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Gregwar\CaptchaBundle;
 
 use Symfony\Component\HttpKernel\Bundle\Bundle;