Bump Java.Runtime.Environment to C# 9

Author: pjcollins

src/Java.Runtime.Environment/Java.Runtime.Environment.csproj

@@ -5,7 +5,7 @@
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <LangVersion>8.0</LangVersion>
+    <LangVersion>9.0</LangVersion>
     <Nullable>enable</Nullable>
     <MSBuildWarningsAsMessages>NU1702</MSBuildWarningsAsMessages>
   </PropertyGroup>