deprecate the base_template_class option

Author: xabbuh

reference/configuration/twig.rst

@@ -62,6 +62,10 @@ base_template_class
 
 **type**: ``string`` **default**: ``Twig\Template``
 
+.. deprecated:: 7.1
+
+    The ``base_template_class`` option is deprecated since Symfony 7.1.
+
 Twig templates are compiled into PHP classes before using them to render
 contents. This option defines the base class from which all the template classes
 extend. Using a custom base template is discouraged because it will make your