Update UpdatePanelUpdateModeVB.aspx (#7986)

Per https://docs.microsoft.com/en-us/dotnet/visual-basic/programming-guide/program-structure/coding-conventions#use-the-isnot-keyword

Author: mathewspete

snippets/visualbasic/VS_Snippets_Atlas/System.Web.UI.UpdatePanel.UpdateMode/VB/UpdatePanelUpdateModeVB.aspx

@@ -7,7 +7,7 @@
 <script runat="server">
     Protected Property LastUpdate() As DateTime
         Get
-            If Not ViewState("LastUpdate") Is Nothing Then
+            If ViewState("LastUpdate") IsNot Nothing Then
                 Return ViewState("LastUpdate")
             Else : Return DateTime.Now()
             End If