Update dependencies from https://github.com/dotnet/winforms build 20210914.18 (#5327)

[release/6.0] Update dependencies from dotnet/winforms

eng/Version.Details.xml

 <?xml version="1.0" encoding="utf-8"?>
 <Dependencies>
   <ProductDependencies>
-    <Dependency Name="Microsoft.Private.Winforms" Version="6.0.0-rc.2.21464.16">
+    <Dependency Name="Microsoft.Private.Winforms" Version="6.0.0-rtm.21464.18">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>f7daead08c7d1f7d15712844d3ad8ff630cfa6f1</Sha>
+      <Sha>66889940e6dd576e0a34f8fb87b6994c9d30fa9e</Sha>
     </Dependency>
-    <Dependency Name="Microsoft.Dotnet.WinForms.ProjectTemplates" Version="6.0.0-rc.2.21464.16">
+    <Dependency Name="Microsoft.Dotnet.WinForms.ProjectTemplates" Version="6.0.0-rtm.21464.18">
       <Uri>https://github.com/dotnet/winforms</Uri>
-      <Sha>f7daead08c7d1f7d15712844d3ad8ff630cfa6f1</Sha>
+      <Sha>66889940e6dd576e0a34f8fb87b6994c9d30fa9e</Sha>
     </Dependency>
     <Dependency Name="System.CodeDom" Version="6.0.0-rc.2.21464.21" CoherentParentDependency="Microsoft.Private.Winforms">
       <Uri>https://github.com/dotnet/runtime</Uri>

eng/Versions.props

@@ -9,7 +9,7 @@
   </PropertyGroup>
   <!-- NuGet Package Versions -->
   <PropertyGroup>
-    <MicrosoftPrivateWinformsVersion>6.0.0-rc.2.21464.16</MicrosoftPrivateWinformsVersion>
+    <MicrosoftPrivateWinformsVersion>6.0.0-rtm.21464.18</MicrosoftPrivateWinformsVersion>
   </PropertyGroup>
   <!-- Packages that come from https://github.com/dotnet/coreclr -->
   <PropertyGroup>