Update langVersion to latest

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/Microsoft.OpenApi.Readers.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
         <TargetFrameworks>netstandard2.0</TargetFrameworks>
-        <LangVersion>9.0</LangVersion>
+        <LangVersion>latest</LangVersion>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
         <PackageIconUrl>http://go.microsoft.com/fwlink/?LinkID=288890</PackageIconUrl>
         <PackageProjectUrl>https://github.com/Microsoft/OpenAPI.NET</PackageProjectUrl>