form_builder: checkbox backward compatibility

templates/element-form/form-partials/checkbox.html.twig

 <div class="checkbox-container">
   <input type="checkbox" name="data[{{ field.name }}]" id="{{ field.name }}"
           {% if field.required|default(false) %}class="required"{% endif %}
-          data-checked-value="{{ field.checked_value }}"
+          data-checked-value="{{ field.checked_value|default('on') }}"
           {% if elementValue or not elementValue and field.defaultvalue == "yes" %}checked="checked"{% endif %}/>
   <label for="{{ field.name }}" class="to-html">{{ field.label|raw }}</label>
 </div>
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