Update overlaycornerradius value (#9119)

Author: harshit7962

src/Microsoft.DotNet.Wpf/src/Themes/PresentationFramework.Fluent/Resources/Variables.xaml

@@ -16,7 +16,7 @@
 
     <system:Double x:Key="ControlContentThemeFontSize">14</system:Double>
     <CornerRadius x:Key="ControlCornerRadius">4,4,4,4</CornerRadius>
-    <CornerRadius x:Key="OverlayCornerRadius">4,4,4,4</CornerRadius>
+    <CornerRadius x:Key="OverlayCornerRadius">8,8,8,8</CornerRadius>
     <CornerRadius x:Key="PopupCornerRadius">8,8,8,8</CornerRadius>
 
     <!--