qt6: Fix type registration

In Qt 6, registration of `QDataStream` streaming operators is done
automatically. Consequently, `qRegisterMetaTypeStreamOperators()` does
no longer exist. Calls to this method have to be removed when porting
to Qt 6.

See https://doc.qt.io/qt-6/qtcore-changes-qt6.html#the-qmetatype-class

src/qt/bitcoin.cpp

@@ -96,7 +96,11 @@ static void RegisterMetaTypes()
     qRegisterMetaType<QMessageBox::Icon>("QMessageBox::Icon");
     qRegisterMetaType<interfaces::BlockAndHeaderTipInfo>("interfaces::BlockAndHeaderTipInfo");
 
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
     qRegisterMetaTypeStreamOperators<BitcoinUnit>("BitcoinUnit");
+#else
+    qRegisterMetaType<BitcoinUnit>("BitcoinUnit");
+#endif
 }
 
 static QString GetLangTerritory()