man: redefine SUPPORT_END= to mean one day earlier

I thought it would be nice to specify the last day of support, because I
thought it'd seem more natural. But in practice this doesn't work well, because
such a truncated timestamp is usually taken to mean midnight that starts the
given date. I.e. 2011-12-13 is a shorthand for 2011-12-13 00:00:00 and not
2011-12-13 23:59:59.999999999999. Let's instead specify that the given date is
the first unsupported day, which is meaningful for humans, and let the computer
treat it as midnight, which gives consistent interpratation.

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