Improve accessibility of panel heading elements

_includes/toc.html

@@ -10,9 +10,7 @@
 <script src="https://cdn.jsdelivr.net/gh/afeld/bootstrap-toc@1.0.1/dist/bootstrap-toc.min.js"></script>
 
 <div id="toc-wrapper" class="pl-0 pr-4 mb-5">
-  <h4 class="pl-3 pt-2 mb-2">
-    {{- site.data.locales[lang].panel.toc -}}
-  </h4>
+  <div class="panel-heading pl-3 pt-2 mb-2">{{- site.data.locales[lang].panel.toc -}}</div>
   <nav id="toc" data-toggle="toc"></nav>
 </div>
 {% endif %}

_includes/trending-tags.html

@@ -37,7 +37,7 @@
 
 {% if trending_tags.size > 0 %}
   <div id="access-tags">
-    <h4 class="text-muted">{{- site.data.locales[lang].panel.trending_tags -}}</h4>
+    <div class="panel-heading">{{- site.data.locales[lang].panel.trending_tags -}}</div>
     <div class="d-flex flex-wrap mt-3 mb-1 mr-3">
 
     {% for tag_name in trending_tags %}

_includes/update-list.html

@@ -26,7 +26,7 @@
 {% if update_list.size > 0 %}
 
   <div id="access-lastmod" class="post">
-    <h4>{{- site.data.locales[lang].panel.lastmod -}}</h4>
+    <div class="panel-heading">{{- site.data.locales[lang].panel.lastmod -}}</div>
     <ul class="post-content pl-0 pb-1 ml-1 mt-2">
       {% for item in update_list %}
         {% assign index = item | split: "::" | last | plus: 0 %}