Update dependencies from https://github.com/dotnet/winforms build 20200518.2 (#3013)

Microsoft.Private.Winforms
 From Version 5.0.0-preview.6.20268.1 -> To Version 5.0.0-preview.6.20268.2
Co-authored-by: Ndotnet-maestro[bot] <dotnet-maestro[bot]@users.noreply.github.com>

eng/Version.Details.xml

 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
-    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20268.1">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20268.2">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>e3f5000b7714c43671f1e0b86110d2993d697238</Sha>
+      <Sha>3607a24b9c43376515932bcda07a3e46be79c305</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20268.1</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20268.2</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>