Always use UTF-8 for decoding GnuPG output on Windows

Since 2.2.28, GnuPG always uses UTF-8 for console output (and input).
For older versions re-add the old code to guess the code page because
the --show-codepages option of gpgconf is only available in GnuPG 2.3.

• GnuPG-bug-id: T5960