Windows is heading toward making UTF-8 the preferred MBCS. As CMake's
internal encoding, `UTF-8` is effectively equivalent to `NONE`, which
was CMake's behavior prior to 3.15's accidental change to `AUTO`.
Behavior of `ENCODING UTF-8` is independent of CMake's internal
encoding, making it in principle a better default than `NONE`.
Add policy CMP0176 for compatibility and to document the default's
history.
Fixes: #26262
