Make GUI incapable of setting tx confirm target of 1

src/qt/forms/sendcoinsdialog.ui

@@ -1064,7 +1064,7 @@
                       <number>0</number>
                      </property>
                      <property name="maximum">
-                      <number>24</number>
+                      <number>23</number>
                      </property>
                      <property name="pageStep">
                       <number>1</number>

src/qt/sendcoinsdialog.cpp

@@ -175,7 +175,7 @@ void SendCoinsDialog::setModel(WalletModel *_model)
         // set the smartfee-sliders default value (wallets default conf.target or last stored value)
         QSettings settings;
         if (settings.value("nSmartFeeSliderPosition").toInt() == 0)
-            ui->sliderSmartFee->setValue(ui->sliderSmartFee->maximum() - model->getDefaultConfirmTarget() + 1);
+            ui->sliderSmartFee->setValue(ui->sliderSmartFee->maximum() - model->getDefaultConfirmTarget() + 2);
         else
             ui->sliderSmartFee->setValue(settings.value("nSmartFeeSliderPosition").toInt());
     }
@@ -241,7 +241,7 @@ void SendCoinsDialog::on_sendButton_clicked()
     if (model->getOptionsModel()->getCoinControlFeatures())
         ctrl = *CoinControlDialog::coinControl;
     if (ui->radioSmartFee->isChecked())
-        ctrl.nConfirmTarget = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 1;
+        ctrl.nConfirmTarget = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 2;
     else
         ctrl.nConfirmTarget = 0;
 
@@ -601,7 +601,7 @@ void SendCoinsDialog::updateGlobalFeeVariables()
 {
     if (ui->radioSmartFee->isChecked())
     {
-        int nConfirmTarget = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 1;
+        int nConfirmTarget = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 2;
         payTxFee = CFeeRate(0);
 
         // set nMinimumTotalFee to 0 to not accidentally pay a custom fee
@@ -646,7 +646,7 @@ void SendCoinsDialog::updateSmartFeeLabel()
     if(!model || !model->getOptionsModel())
         return;
 
-    int nBlocksToConfirm = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 1;
+    int nBlocksToConfirm = ui->sliderSmartFee->maximum() - ui->sliderSmartFee->value() + 2;
     int estimateFoundAtBlocks = nBlocksToConfirm;
     CFeeRate feeRate = mempool.estimateSmartFee(nBlocksToConfirm, &estimateFoundAtBlocks);
     if (feeRate <= CFeeRate(0)) // not enough data => minfee