pgsql: Don't recommend upgrading to latest available Windows SDK

Don't recommend upgrading to latest available Windows SDK

We only support up to version 7.0, so don't recommend
upgrading past it. The rest of the documentation around this
was already updated, but one spot was missed.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/2367da886d4ab903c7bf5037b363ca10489cdf85

Modified Files
--------------
doc/src/sgml/install-windows.sgml | 7 ++++---
1 files changed, 4 insertions(+), 3 deletions(-)