Bump MIN_WINNT to 0x0600 (Vista) as minimal runtime in 16~

Hi all,

On the thread about the removal of VS 2013, Jose (in CC) has mentioned
that bumping MIN_WINNT independently would make sense, as the
simplication of locales would expose under MinGW some code for
GetLocaleInfoEx():
https://www.postgresql.org/message-id/CAC+AXB3himFH+-pGRO1cYju6zF2hLH6VmwPbf5RAytF1UBm_nw@mail.gmail.com

Attached is a patch to set MIN_WINNT, the minimal version of Windows
allowed at run-time to 0x0600 for all environments, aka Vista. This
results in removing the support for XP at run-time when compiling with
anything else than VS >= 2015 (VS 2013, MinGW, etc.). We could cut
things more, I hope, but this bump makes sense in itself with the
business related to locales.

What I would like to do is to apply that at the beginning of the dev
cycle for v16, in parallel of the removal of VS 2013. This move is
rather independent of the other thread, which is why I am spawning a
new one here. And it is better than having to dig into the other
thread for a change like that.

Thoughts or opinions?
--
Michael