Don't serialize obsolete properties (#12838)

Fixes #12836

Author: Tanya-Solyanik

src/System.Windows.Forms/src/System/Windows/Forms/Control.cs

@@ -12858,6 +12858,7 @@ internal ToolStripControlHost? ToolStripControlHost
         UrlFormat = Obsoletions.SharedUrlFormat)]
     [EditorBrowsable(EditorBrowsableState.Never)]
     [Browsable(false)]
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
     public virtual ContextMenu ContextMenu
     {
         get;
@@ -12875,6 +12876,7 @@ public virtual ContextMenu ContextMenu
 
     [EditorBrowsable(EditorBrowsableState.Never)]
     [Browsable(false)]
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
     public event EventHandler ContextMenuChanged
     {
         add { }
@@ -12891,6 +12893,7 @@ public event EventHandler ContextMenuChanged
         UrlFormat = Obsoletions.SharedUrlFormat)]
     [EditorBrowsable(EditorBrowsableState.Never)]
     [Browsable(false)]
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
     protected virtual void OnContextMenuChanged(EventArgs e) { }
 #nullable enable
 }

src/System.Windows.Forms/src/System/Windows/Forms/Form.cs

@@ -7140,6 +7140,7 @@ protected override void WndProc(ref Message m)
         UrlFormat = Obsoletions.SharedUrlFormat)]
     [EditorBrowsable(EditorBrowsableState.Never)]
     [Browsable(false)]
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
     public MainMenu Menu
     {
         get;
@@ -7156,6 +7157,7 @@ public MainMenu Menu
         UrlFormat = Obsoletions.SharedUrlFormat)]
     [EditorBrowsable(EditorBrowsableState.Never)]
     [Browsable(false)]
+    [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
     public MainMenu MergedMenu
     {
         get;