Fix datepicker not refreshing format when date format changes (#18599)

* fix DatePicker not refresh when  MonthFormat or YearFormat changed

* use SetSelectedDateText instead

Author: seishinkouki

src/Avalonia.Controls/DateTimePickers/DatePicker.cs

@@ -293,6 +293,10 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
                 var (oldValue, newValue) = change.GetOldAndNewValue<DateTimeOffset?>();
                 OnSelectedDateChanged(this, new DatePickerSelectedValueChangedEventArgs(oldValue, newValue));
             }
+            else if (change.Property == MonthFormatProperty || change.Property == YearFormatProperty || change.Property == DayFormatProperty)
+            {
+                SetSelectedDateText();
+            }
         }
 
         private void OnDismissPicker(object? sender, EventArgs e)

src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs

@@ -312,6 +312,10 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
             {
                 OnDateRangeChanged(this, change);
             }
+            else if (change.Property == MonthFormatProperty || change.Property == YearFormatProperty || change.Property == DayFormatProperty)
+            {
+                InitPicker();
+            }
         }
 
         protected override void OnKeyDown(KeyEventArgs e)