VS: Unable to choose Windows 8.1 SDK version in VS 16 2019

Work in !2962 (merged) and discussion in #18923 (closed) identified that when no WindowsTargetPlatformVersion is written to a .vcxproj file then VS 2019 chooses "10.0 latest" instead of "8.1" as VS 2017 did.

This means that an explicit value may be needed in order to get 8.1.

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