windows: Set _WIN32_WINNT to 0x0601 (Windows 7)

Summary:
Was set to 0x0501 (XP) which is no longer supported.

Backport of core PR14922.

Test Plan:

cmake -GNinja ..\
  -DCMAKE_TOOLCHAIN_FILE=../cmake/platforms/Win64.cmake \
  -DBUILD_BITCOIN_SEEDER=OFF
ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7741