Bump C#: 9.0->11.0

Author: AndreyAkinshin

build/common.props

@@ -33,7 +33,7 @@
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(IsVisualBasic)' != 'true' AND '$(IsFsharp)' != 'true' ">
-    <LangVersion>9.0</LangVersion>
+    <LangVersion>11.0</LangVersion>
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(VersionPrefix)' == '' ">