QSqlQuery: Do not use deleted constructor in docs

QVariant(QMetaType::Type) is deleted since Qt 5 (sic!). Suggest usage of
the constructor taking a meta-type instead.

Pick-to: 6.2 6.3 6.4
Change-Id: I50949dac9dfc35c80108debdeabc791afe0998b6
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/sql/kernel/qsqlquery.cpp

@@ -1047,8 +1047,8 @@ bool QSqlQuery::exec()
 
   To bind NULL values, a null QVariant of the relevant type has to be
   added to the bound QVariantList; for example, \c
-  {QVariant(QMetaType::QString)} should be used if you are using
-  strings.
+  {QVariant(QMetaType::fromType<QString>())} should be used if you are
+  using strings.
 
   \note Every bound QVariantList must contain the same amount of
   variants.