Add C# 12 (see #245)

[konrad-gora]

• Name: C#
• Version: 12
• Release Note/Changelog: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-12 this one is also valid as there is no runner for C# 11: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-11

C# 12 was introduced with .NET 8 which is LTS version (like .NET 6 and C# 10)

It would be good to have in .csproj these entries:

    <PropertyGroup>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

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:+1: reaction might help to get this request prioritized.

[hobovsky]

Possibly supersedes #245

[kazk]

Decided to update #245 instead so we don't lose the :+1: count for the newer C#. We don't always prioritize based on the :+1: count, but it's one of the major factors used to decide what to work on.