Update 'dotnet octo' tool for .NET 5

[Luca Giordano]

I'm currently leveraging the tool in a /dotnet/sdk:5.0 container and unfortunately getting the following error ...

It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '3.1.0' was not found.

• The following frameworks were found:
  5.0.1 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

[Luca Giordano]

Duplicate of OctopusDeploy/Issues#6704

[Jeremy Miller]

Duplicate as stated above. Closing this one and put both references in 6704.