minor #9050 [TwigBundle] Fix default value of strict_variables (yceruto)

This PR was merged into the 2.7 branch.

Discussion
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[TwigBundle] Fix default value of strict_variables

See https://github.com/symfony/symfony/blob/d78a98d793c2846d6decbff652d63f981e4f32f4/src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php#L177

However, I'm proposing revert it since 4.1 and fix it inside the configuration section, see #9049

Commits
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3888217 Fix default value of strict_variables

Author: javiereguiluz

reference/configuration/twig.rst

@@ -439,7 +439,7 @@ by Symfony. Besides, it simplifies how you refer to those templates:
 strict_variables
 ~~~~~~~~~~~~~~~~
 
-**type**: ``boolean`` **default**: ``'%kernel.debug%'``
+**type**: ``boolean`` **default**: ``false``
 
 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