minor #14574 [Twig] Fix strict_variables default value since Symfony 5 (ogizanagi)

This PR was merged into the 5.1 branch.

Discussion
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[Twig] Fix strict_variables default value since Symfony 5

The default was changed as of Symfony 5 https://github.com/symfony/symfony/blob/494ef421c554a78b38c6779c4b7deb9a20d89923/UPGRADE-5.0.md#twigbundle

Commits
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74de492 Fix strict_variables default value since Symfony 5

Author: javiereguiluz

reference/configuration/twig.rst

@@ -363,7 +363,7 @@ Read more about :ref:`template directories and namespaces <templates-namespaces>
 strict_variables
 ~~~~~~~~~~~~~~~~
 
-**type**: ``boolean`` **default**: ``false``
+**type**: ``boolean`` **default**: ``%kernel.debug%``
 
 If set to ``true``, Symfony shows an exception whenever a Twig variable,
 attribute or method doesn't exist. If set to ``false`` these errors are ignored