bug symfony#26328 [Form][WCAG] Fixed HTML errors (Nyholm)

This PR was squashed before being merged into the 3.4 branch (closes symfony#26328).

Discussion
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[Form][WCAG] Fixed HTML errors

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

According to my friend and WCAG2 expect [Sandra](https://twitter.com/sandrability):

> Fix your HTML errors.

Should I fix the same issues on other forms as well? Or could someone claim that is a BC break?

Commits
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fe66819 [Form][WCAG] Fixed HTML errors

Author: fabpot

src/Symfony/Bridge/Twig/Resources/views/Form/bootstrap_4_layout.html.twig

@@ -103,6 +103,10 @@
     {% if type is not defined or type != 'hidden' %}
         {%- set attr = attr|merge({class: (attr.class|default('') ~ ' form-control' ~ (type|default('') == 'file' ? '-file' : ''))|trim}) -%}
     {% endif %}
+    {%- if type is defined and (type == 'range' or type == 'color') %}
+        {# Attribute "required" is not supported #}
+        {%- set required = false -%}
+    {% endif %}
     {{- parent() -}}
 {%- endblock form_widget_simple %}