Update dependencies from https://github.com/dotnet/winforms build 20200511.7 (#2985)

- Microsoft.Private.Winforms: 5.0.0-preview.6.20261.4 -> 5.0.0-preview.6.20261.7
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.6.20261.4">
+    <Dependency Name="Microsoft.Private.Winforms" Version="5.0.0-preview.6.20261.7">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>92eb28120a54da9279887cbd733f906a25ca196f</Sha>
+      <Sha>c6348344ca268122d6ffe652115ee569886a55d7</Sha>
     </Dependency>
   </ProductDependencies>
   <ToolsetDependencies>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20261.4</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>5.0.0-preview.6.20261.7</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>