Directory.Build.props: Upgraded to C# 9.0
diff --git a/Directory.Build.props b/Directory.Build.props
index 8cd9ea7..e36781d 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -22,7 +22,7 @@
   <Import Project="build\Dependencies.props" />
 
   <PropertyGroup>
-    <LangVersion>8.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
   </PropertyGroup>
   
   <!-- IMPORTANT: When these values are changed, the CI counter number should also be reset. -->