build: remove implied support for win 2012 not R2

Raising this primarily for the discussion of whether to remove the implied support for Win 2012 not R2. The basis for this is that CI does not run on Win 2012 not R2, and some failures have been deemed specific to this platform.

See:

• https://github.com/nodejs/node/issues/18177
• https://github.com/nodejs/node/issues/18454

/cc @gibfahn

Checklist

• documentation is changed or added
• commit message follows commit guidelines