Xhtml5Sink.date_() emits uncommon meta name

### Affected version

2.0.0

### Bug description

The date is emitted as `` which is not a known name according to https://html.spec.whatwg.org/multipage/semantics.html#the-meta-element. Although https://maven.apache.org/doxia/doxia/doxia-sink-api/apidocs/org/apache/maven/doxia/sink/Sink.html#date-- is quite fuzzy about the semantics I think instead emitting one of the meta extensions from https://wiki.whatwg.org/wiki/MetaExtensions makes more sense.

Locate Xhtml5Sink.date_() and inspect how it emits the date meta element. Check the HTML meta-element and MetaExtensions references linked in the issue to determine the appropriate metadata name, then update the output and add or adjust the relevant test if one is present. Done means the date uses an accepted meta extension rather than name="date".

Written by the indexing model from the issue text.