Commit 823abcd2 introduced a call to KIO::statDetails(), which only exists
since KIO 5.69.0.
But the specified minimum KDE Frameworks version is 5.55.0, so switch back
to using KIO::stat() when building with KIO < 5.69.0.

Also, pass KIO::StatNoDetails to KIO::statDetails() to not request unnecessary
details, as the intention is to just check whether the file exists.