cmake-gui: Support non-ASCII chars on Windows with Qt6

Since commit baead1e2a8 (Encoding: Remove option to use ANSI code page
internally, 2016-11-01, v3.8.0-rc1~358^2), we always use UTF-8 strings
internally.  Using fromLocal8Bit/toLocal8Bit + QTextCodec is no longer
needed.  Convert to fromStdString/toStdString instead, which should work
both with Qt5 and Qt6.

Fixes: #23565