Update csproj with latest c# version & enabled nullable

Author: scorteanucosmin

src/Oxide.Core.csproj

@@ -13,8 +13,10 @@
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <PackageProjectUrl>$(RepositoryUrl)</PackageProjectUrl>
     <TargetFrameworks>net35;net40;net45;net46;net48;netstandard2.0;netstandard2.1</TargetFrameworks>
+      <LangVersion>Latest</LangVersion>
     <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
     <NoWarn>NU1701</NoWarn>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Lib.Harmony" Version="2.3.1.1" />