Fix Qt 6 build of ProjectExplorer and targets & build systems

QStringRef is gone.
qsizetype vs int.
QMultiHash::insertMulti is gone.
QXmlStreamWriter writes UTF-8 by default.

Task-number: QTCREATORBUG-24098
Change-Id: Id217e40a1f17993a84fc725976e9cb84618b0580
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmakeprojectmanager/customwidgetwizard/classdefinition.cpp

@@ -79,7 +79,7 @@ static inline QString xmlFromClassName(const QString &name)
     if (!name.isEmpty()) {
         rc += name.left(1).toLower();
         if (name.size() > 1)
-            rc += name.midRef(1);
+            rc += name.mid(1);
     }
     rc += QLatin1String("\">\n</widget>\n");
     return rc;