bug #26870 Add d-block to bootstrap 4 alerts (Normunds)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes #26870).

Discussion
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Add d-block to bootstrap 4 alerts

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #26869
| License       | MIT
| Doc PR        | N/A

Commits
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40bd8bd Add d-block to bootstrap 4 alerts

Author: nicolas-grekas

Resources/views/Form/bootstrap_4_layout.html.twig

@@ -267,7 +267,7 @@
 
 {% block form_errors -%}
     {%- if errors|length > 0 -%}
-        <span class="{% if form is not rootform %}invalid-feedback d-block{% else %}alert alert-danger{% endif %}">
+        <span class="{% if form is not rootform %}invalid-feedback{% else %}alert alert-danger{% endif %} d-block">
             {%- for error in errors -%}
                 <span class="mb-0 d-block">
                     <span class="initialism form-error-icon badge badge-danger">{{ 'Error'|trans({}, 'validators') }}</span> <span class="form-error-message">{{ error.message }}</span>