[TCLCORE] Change to tclWinPort.h for supported Windows versions

[<apn...@ya...>]

As per TIP 715 (platform support), 9.1 will only support Windows 10 and
later versions.

 

I am modifying the _WIN32_WINNT definition in tclWinPort.h accordingly.
Windows SDK's will also have to be recent versions as specified in the TIP.

 

Just FYI so no one is surprised.

 

/Ashok