Use generic div element for tabspanel

Sections when they have an accessible name create landmarks.

> The use of region landmarks should be limited, as too many landmarks can dilute their usefulness in quickly navigating to important areas of a page. If you find there are many landmarks, especially generic regions existing in a single page, then the page’s structure should be reexamined.

- https://www.scottohara.me/blog/2018/03/03/landmarks.html

Also, when the component has not be enhanced these sections do not have
accessible names so it means they are the equivalent of a div already.

When they're enhanced they're given a role of tabpanel, so they're no
longer a section.

Given this I'm not sure it was ever useful and by making them generic
divs we can avoid HTML validators warning about this component.

Closes #1339

src/govuk/components/tabs/template.njk

@@ -24,9 +24,9 @@
   {% for item in params.items %}
     {% if item %}
       {% set id = item.id if item.id else idPrefix + "-" + loop.index %}
-      <section class="govuk-tabs__panel{% if loop.index > 1 %} govuk-tabs__panel--hidden{% endif %}" id="{{ id }}"{% for attribute, value in item.panel.attributes %} {{attribute}}="{{value}}"{% endfor %}>
+      <div class="govuk-tabs__panel{% if loop.index > 1 %} govuk-tabs__panel--hidden{% endif %}" id="{{ id }}"{% for attribute, value in item.panel.attributes %} {{attribute}}="{{value}}"{% endfor %}>
         {{ item.panel.html | safe if item.panel.html else item.panel.text }}
-      </section>
+      </div>
     {% endif %}
   {% endfor %}
 </div>