N°6934 - Symfony 6.4 - upgrade Symfony bundles to 6.4 (#580)

* Update Symfony lib to version ~6.4.0
* Update code missing return type
* Add an iTop general configuration entry to store application secret (Symfony mandatory parameter)
* Use dependency injection in ExceptionListener & UserProvider classes

lib/symfony/string/Resources/functions.php

@@ -31,7 +31,7 @@ if (!\function_exists(s::class)) {
      */
     function s(?string $string = ''): AbstractString
     {
-        $string = $string ?? '';
+        $string ??= '';
 
         return preg_match('//u', $string) ? new UnicodeString($string) : new ByteString($string);
     }