N°4517 - PHP 8.1: Fix return type for classes implementing ArrayAccess

Return type cannot be set to mixed as the code need to be compatible with PHP 7.2+

datamodels/2.x/itop-portal-base/portal/src/Twig/AppVariable.php

@@ -73,6 +73,7 @@ class AppVariable implements ArrayAccess
     /**
      * @inheritDoc
      */
+	#[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
 	    if ($this->container->hasParameter($offset)) {