Update release/6.0 branding to GA (#5332)

* Update release/6.0 branding to GA

* Use LangVersion 9 until build pool machines are updated.

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>rc</PreReleaseVersionLabel>
-    <PreReleaseVersionIteration>2</PreReleaseVersionIteration>
+    <PreReleaseVersionLabel>rtm</PreReleaseVersionLabel>
+    <PreReleaseVersionIteration></PreReleaseVersionIteration>
     <SystemIOPackagingVersion>6.0.0-rc.2.21464.21</SystemIOPackagingVersion>
     <SystemResourcesExtensionsVersion>6.0.0-rc.2.21464.21</SystemResourcesExtensionsVersion>
   </PropertyGroup>

eng/WpfArcadeSdk/Sdk/Sdk.props

@@ -15,7 +15,10 @@
   <PropertyGroup>
     <GenXmlStringTable>$(WpfArcadeSdkToolsDir)GenXmlStringTable.pl</GenXmlStringTable>
     <LangVersion Condition="'$(LangVersion)'=='' and ($(PreReleaseVersionLabel.Contains('rc')) or $(PreReleaseVersionLabel.Contains('preview')))">preview</LangVersion>
-    <LangVersion Condition="'$(LangVersion)'==''">10</LangVersion>
+    <!--
+        LangVersion 10 can't be set until the build pool machines are updated.  Use version 9 for now.
+    -->
+    <LangVersion Condition="'$(LangVersion)'==''">9</LangVersion>
     <CLSCompliant Condition="'$(CLSCompliant)'==''">true</CLSCompliant>
     <IncludeDllSafeSearchPathAttribute Condition="'$(IncludeDllSafeSearchPathAttribute )'==''">true</IncludeDllSafeSearchPathAttribute>