Use latest C# version

Rather than preview.

Author: martincostello

Directory.Build.props

@@ -8,7 +8,7 @@
     <EnableNETAnalyzers>true</EnableNETAnalyzers>
     <GenerateDocumentationFile>false</GenerateDocumentationFile>
     <ImplicitUsings>enable</ImplicitUsings>
-    <LangVersion>preview</LangVersion>
+    <LangVersion>latest</LangVersion>
     <NeutralLanguage>en-US</NeutralLanguage>
     <Nullable>enable</Nullable>
     <PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>