UI: Don't use QTextStream::setCodec in Qt6

In Qt6, QTextStream::setCodec has been replaced by
QTextStream::setEncoding. However, all text is assumed to be UTF-8, so
we don't need to specify UTF-8 in Qt6.

https://doc.qt.io/qt-5/qtextstream.html#setCodec-1
https://doc.qt.io/qt-6/qtextstream.html#setEncoding

UI/log-viewer.cpp

@@ -101,9 +101,7 @@ void OBSLogViewer::InitLog()
 
 	if (file.open(QIODevice::ReadOnly)) {
 		QTextStream in(&file);
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
-		in.setEncoding(QStringConverter::Utf8);
-#else
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 		in.setCodec("UTF-8");
 #endif