Update branding to Preview6

Author: dougbu

eng/Versions.props

@@ -9,7 +9,7 @@
     <AspNetCoreMajorVersion>5</AspNetCoreMajorVersion>
     <AspNetCoreMinorVersion>0</AspNetCoreMinorVersion>
     <AspNetCorePatchVersion>0</AspNetCorePatchVersion>
-    <PreReleaseVersionIteration>5</PreReleaseVersionIteration>
+    <PreReleaseVersionIteration>6</PreReleaseVersionIteration>
     <!--
         When StabilizePackageVersion is set to 'true', this branch will produce stable outputs for 'Shipping' packages
     -->