How to use C# 8.0 with Godot 3.3.2?

I am trying to use the System.Range and System.Index classes with new C# v8.0 construct, see (here). But, I cannot make it work. I have tried under Windows with .NET 5.0 and under Ubuntu with Mono 6 but I got the same result:

First I have changed the .csproj file like that:

<Project Sdk="Godot.NET.Sdk/3.3.0">

Note the <LangVersion> set at 8.0. With that parameter, I can now build the project with this kind of snippet:

int() nb = {1, 2, 3};

But at runtime, I got this error:
Predefined type 'System.Range' is not defined or imported

EDIT: To answer the comments.

I had to add using System.Runtime to make the project able to build.

I had tried to change target framework to .net5.0 (that was only possible on Windows). The project was still able to build but the error at runtime was different:

E 0:00:01.563   debug_send_unhandled_exception_error: System.TypeLoadException: Could not resolve type with token 01000010 from typeref (expected class 'System.Range' in assembly 'System.Runtime, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a')
  <C++ Error>   Unhandled exception
  <C++ Source>  modules/mono/mono_gd/gd_mono_utils.cpp:423 @ debug_send_unhandled_exception_error()

But this answer made me think that the Mono assemblies used at runtime needs net472.

Another hint is that an important Godot project that claims to use C# v8.0, the beautiful Thrive, sets net472 and 8.0 in their config file (see here)