Retarget to Windows 7

[jbanaszczyk]

Form ICU: icu\icu4c\source\allinone\Build.Windows.ProjectConfiguration.props

  <PropertyGroup>
    <!-- This is the default SDK target. -->
    <!-- Note that the Windows 8.1 SDK is backwards compatible down-level to Windows 7, so
         setting this to 8.1 does not actually imply targeting Windows 8.1. -->
    <WindowsTargetPlatformVersion>8.1</WindowsTargetPlatformVersion>
  </PropertyGroup>
  <PropertyGroup>
    <!-- We need to explicitly set the target version to Windows 7. -->
    <Win32_WinNTVersion>0x0601</Win32_WinNTVersion>
  </PropertyGroup>