minor #13594 Adding an if condition on a for tag is deprecated in Twig 2.10. (BooleanType)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Adding an if condition on a for tag is deprecated in Twig 2.10.

I've got an error, running example from the docs. Here is the explanation: https://twig.symfony.com/doc/2.x/deprecated.html#tags
I propose to use a filter filter or an “if” condition inside the “for” body instead in this docs.

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Commits
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ae6ae33 Adding an if condition on a for tag is deprecated in Twig 2.10.

Author: javiereguiluz

form/create_custom_field_type.rst

@@ -405,7 +405,7 @@ libraries are used in your application:
 
     {# templates/form/custom_types.html.twig #}
     {% block postal_address_row %}
-        {% for child in form.children if not child.rendered %}
+        {% for child in form.children|filter(v => not v.rendered) -%}
             <div class="form-group">
                 {{ form_label(child) }}
                 {{ form_widget(child) }}