Update NuKeeper.csproj to enable .NET 5.0 environments to use the tool

[rajbos]

Update to support .NET 5.0 based environments

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Adds support in the NuKeeper project for usage of the dotnet global tool in a pure .NET 5.0 environment.

:arrow_heading_down: What is the current behavior?

Running in a docker container with only .NET 5.0 installed, this is the current output:

Tool 'nukeeper' (version '0.34.0') was successfully installed.
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:
      5.0.0 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]

So we cannot use NuKeeper without also installing .NET Core 3.1

:new: What is the new behavior (if this is a feature change)?

With this new target framework version dotnet tool install will recognize the correct TFM to use and fix the issue.

:boom: Does this PR introduce a breaking change?

I don't think so :-)

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

:thinking: Checklist before submitting

• [x] All projects build
• [ ] Relevant documentation was updated

[rajbos]

Unfortunately you'll need to update the build pipelines as well to use a new base image. Would love to help if needed 👍

[MarcBruins]

@rajbos Think #1084 also consumes does the same as this PR, i would prefer merging that one

[rajbos]

Great, hope to see the .NET 5.0 support soon :-)