Update Placeholder properties to use TextBox as owner in NumericUpDown & ComboBox (#20929)

* feat: update PlaceholderText and PlaceholderForeground properties to use TextBox as owner in NumericUpDown and ComboBox.

* chore: disable warning.

src/Avalonia.Controls/ComboBox.cs

@@ -62,13 +62,13 @@ namespace Avalonia.Controls
         /// Defines the <see cref="PlaceholderText"/> property.
         /// </summary>
         public static readonly StyledProperty<string?> PlaceholderTextProperty =
-            AvaloniaProperty.Register<ComboBox, string?>(nameof(PlaceholderText));
+            TextBox.PlaceholderTextProperty.AddOwner<ComboBox>();
 
         /// <summary>
         /// Defines the <see cref="PlaceholderForeground"/> property.
         /// </summary>
         public static readonly StyledProperty<IBrush?> PlaceholderForegroundProperty =
-            AvaloniaProperty.Register<ComboBox, IBrush?>(nameof(PlaceholderForeground));
+            TextBox.PlaceholderForegroundProperty.AddOwner<ComboBox>();
 
         /// <summary>
         /// Defines the <see cref="HorizontalContentAlignment"/> property.

src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs

@@ -112,7 +112,9 @@ namespace Avalonia.Controls
         /// Defines the <see cref="PlaceholderText"/> property.
         /// </summary>
         public static readonly StyledProperty<string?> PlaceholderTextProperty =
-            AvaloniaProperty.Register<NumericUpDown, string?>(nameof(PlaceholderText));
+#pragma warning disable AVP1013
+            TextBox.PlaceholderTextProperty.AddOwner<NumericUpDown>();
+#pragma warning restore AVP1013
 
         /// <summary>
         /// Defines the <see cref="Watermark"/> property.