Replace deprecated text-muted class with text-body-secondary

app/views/issues/_reports.html.erb

@@ -4,7 +4,7 @@
       <%= link_to user_thumbnail(report.user), report.user %>
     </div>
     <div class="col">
-      <p class="text-muted">
+      <p class="text-body-secondary">
         <%= t ".reported_by_html", :category => report.category,
                                    :user => link_to(report.user.display_name, report.user),
                                    :updated_at => tag.time(l(report.updated_at.to_datetime, :format => :friendly),