Update to .NET 5

[dhirschfeld]

https://dotnetcli.blob.core.windows.net/dotnet/Runtime/Current/latest.version

[dhirschfeld]

• https://dotnetcli.azureedge.net/dotnet/Sdk/5.0.100/dotnet-sdk-5.0.100-win-x64.zip
• https://dotnetcli.azureedge.net/dotnet/Sdk/5.0.100/dotnet-sdk-5.0.100-linux-x64.tar.gz
• https://dotnetcli.azureedge.net/dotnet/Sdk/5.0.100/dotnet-sdk-5.0.100-osx-x64.tar.gz