[3.13] Correctly fold unknown-8bit originating from encoded words. (GH-142517) (#143147)

The unknown-8bit trick was designed to deal with unknown bytes in an
ASCII message, and it works fine for that.  However, I also tried to
extend it to handle bytes that can't be decoded using the charset
specified in an encoded word, and there it fails because there can be
other non-ASCII characters that were *successfully* decoded.  The fix is
simple: do the unknown-8bit encoding using the utf-8 codec.  This is
especially appropriate since anyone trying to do recovery on an unknown
byte string will probably attempt utf-8 first.
(cherry picked from commit 1e17ccd030a2285ad53db5952360fffa33a8a877)

Co-authored-by: R. David Murray <rdmurray@bitdance.com>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>