Incompatibility with newer dotnet SDKs

Why there's a lock into a specific dotnet cli version in the global.json ?
I think global.json could be improved adding roll forward (ref https://docs.microsoft.com/en-us/dotnet/core/tools/global-json?tabs=netcore3x#rollforward )

Reproduction steps

• Have lastest dotnet SDK - 3.1.301
  • Reference: https://dotnet.microsoft.com/download/dotnet-core/3.1
• Try to build with dotnet core
  • dotnet build

Expected behavior

• libgit2sharp builds

Actual behavior

• Get error message
  • A compatible installed .NET Core SDK for global.json version [3.1.201] from [github/libgit2sharp/global.json] was not found

Version of LibGit2Sharp (release number or SHA1)

• 7fc4be5193dbdd08538b4b150332b5a73770e0f6

Operating system(s) tested; .NET runtime tested

• Ubuntu 18.04
• dotnet sdk 3.1.301

从仓库的 global.json 开始，该文件固定使用 .NET Core SDK 3.1.201，并将其设置与文档中说明的 roll-forward 行为进行比较。使用 dotnet SDK 3.1.301 重现失败并运行 dotnet build；完成标准是项目能够使用这个更新的 SDK 构建，而不会出现版本不兼容错误。