Please update the C# version and shift to native dotnet core sdk instead of mono

[md-redwan-hossain]

In LightOJ, the current C# version stands at 6.8, built on Mono, which unfortunately lacks compatibility with several useful C# features. Presently, Microsoft provides native support for C# and .NET Core on Linux. Notably, Codeforces employs C# 10 with .NET Core SDK 6 LTS. To enhance compatibility and benefit from the latest advancements, it would be highly advantageous for LightOJ to upgrade to at least the same version as Codeforces (dotnet core sdk 6 LTS) or consider adopting an even more recent version, such as dotnet core 8 LTS, if possible. This upgrade would greatly contribute to a more robust and feature-rich C# environment on LightOJ.