Replace Bootstrap 2 label classes with Bootstrap 5 badges

The legend used label/label-important/label-info, which do not exist
in Bootstrap 5.2.3, so the badges rendered unstyled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

app/views/layouts/application.html.erb

@@ -37,10 +37,10 @@
 
         <div class="alert alert-info">
           <div>
-            F: <span class="label label-important">failure</span>
-            E: <span class="label label-important">error</span>
-            S: <span class="label label-info">skip</span>
-            failed: <span class="label label-important">critical failures</span>
+            F: <span class="badge text-bg-danger">failure</span>
+            E: <span class="badge text-bg-danger">error</span>
+            S: <span class="badge text-bg-info">skip</span>
+            failed: <span class="badge text-bg-danger">critical failures</span>
           </div>
           <div>
             see also <a href="https://rubyci.org/coverage">Code coverage report</a>,