[boost] [winapi] Bumping default target to Windows 7?

Hi, I was thinking about bumping the default target Windows version to 7 in Boost.WinAPI. The current default is Vista, which EOLed in April 2017. There is not much difference from the available API standpoint (in Boost.WinAPI this would enable a few constants related to process management), but there may be some interesting extensions that could be useful to libraries. Note that this will not preclude targeting older Windows versions, just as it doesn't preclude targeting XP or 2000 now. Users will still be able to specify the target Windows version as they can now. Note also that I'm not planning this for 1.67, only 1.68. Any objections/thoughts?