[DOCS] release date formatting

It has been pointed out a while ago that the "release date" formatting
in the release notes is too flamboyant, more so under the new stylesheets.

Here is a patch to make the formatting a bit more subdued.  (Obviously,
this needs to be expanded to older release notes as well.)

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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