qt: Update deprecated enum value

This change is preparation for Qt 6, and it fixes an experimental build
with Qt 6.2.4.
The `Qt::ItemIsTristate` value has been deprecated since 5.6.0 (see
ae8406d82f541f6d9112bdac192e5e4e114d56aa upstream commit).

Author: hebasto

src/qt/coincontroldialog.cpp

@@ -588,7 +588,7 @@ void CoinControlDialog::updateView()
     ui->treeWidget->setEnabled(false); // performance, otherwise updateLabels would be called for every checked checkbox
     ui->treeWidget->setAlternatingRowColors(!treeMode);
     QFlags<Qt::ItemFlag> flgCheckbox = Qt::ItemIsSelectable | Qt::ItemIsEnabled | Qt::ItemIsUserCheckable;
-    QFlags<Qt::ItemFlag> flgTristate = Qt::ItemIsSelectable | Qt::ItemIsEnabled | Qt::ItemIsUserCheckable | Qt::ItemIsTristate;
+    QFlags<Qt::ItemFlag> flgTristate = Qt::ItemIsSelectable | Qt::ItemIsEnabled | Qt::ItemIsUserCheckable | Qt::ItemIsAutoTristate;
 
     int nDisplayUnit = model->getOptionsModel()->getDisplayUnit();