bug symfony#29115 Change button_widget class to btn-primary (neFAST)

This PR was submitted for the master branch but it was squashed and merged into the 3.4 branch instead (closes symfony#29115).

Discussion
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Change button_widget class to btn-primary

FIX symfony#29114

| Q             | A
| ------------- | ---
| Branch?       | master for features / 2.8 up to 4.1 for bug fixes <!-- see below -->
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | symfony#29114   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Change button_widget class to btn-primary in Bootstrap 4 twig template

Commits
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59003bf Change button_widget class to btn-primary

Author: nicolas-grekas

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

@@ -137,6 +137,11 @@
     {{- parent() -}}
 {%- endblock button_widget %}
 
+{% block submit_widget -%}
+    {%- set attr = attr|merge({class: (attr.class|default('btn-primary'))|trim}) -%}
+    {{- parent() -}}
+{%- endblock submit_widget %}
+
 {% block checkbox_widget -%}
     {%- set parent_label_class = parent_label_class|default(label_attr.class|default('')) -%}
     {%- if 'checkbox-custom' in parent_label_class -%}