templates: Use standardized value for name attribute in <meta>

In commit bbaaf25 (templates: Add date and author to head metadata,
2020-04-10), a <meta> tag containing the publication date was added for
unknown reasons. It is unclear where this line is from, as the 'scheme'
attribute is not even part of the HTML5 specification. Furthermore, the
value of "date" for the 'name' attribute is not standardized.

Use "dc:date" as per the Dublin Core Metadata Initiative [1] instead of
"date" and remove the 'scheme' attribute fully.

[1] https://www.dublincore.org/specifications/dublin-core/dcmi-terms/#http%3a%2f%2fpurl.org%2fdc%2fterms%2fdate

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