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

從 repository 的 global.json 開始，該檔案固定使用 .NET Core SDK 3.1.201，並將其設定與文件中說明的 roll-forward 行為進行比較。使用 dotnet SDK 3.1.301 重現失敗並執行 dotnet build；完成標準是專案能夠使用這個較新的 SDK 建置，而不會出現版本不相容錯誤。