Update branding after branching release/6.0-preview1 (#4083)

eng/Versions.props

@@ -2,8 +2,8 @@
 <Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <VersionPrefix>6.0.0</VersionPrefix>
-    <PreReleaseVersionLabel>alpha</PreReleaseVersionLabel>
-    <PreReleaseVersionIteration>1</PreReleaseVersionIteration>
+    <PreReleaseVersionLabel>preview</PreReleaseVersionLabel>
+    <PreReleaseVersionIteration>2</PreReleaseVersionIteration>
     <SystemIOPackagingVersion>6.0.0-preview.1.21074.3</SystemIOPackagingVersion>
     <SystemResourcesExtensionsVersion>6.0.0-preview.1.21074.3</SystemResourcesExtensionsVersion>
   </PropertyGroup>