The `pubdate` attribute is not valid for modern HTML(5).

The [h-entry microformat](http://microformats.org/wiki/h-entry) describes
an alternative way to achieve the same thing: a `dt-published` class name.

Patch: <https://github.com/jmtd/ikiwiki/commit/a137103d3004cc8cec42459205684ec48af0ca11>
[[!tag patch]]
[[!template id=gitbranch branch=jon/html5-no-pubdate-attribute author="[[Jon]]"]]
*—[[Jon]], 2020-10-06*