Badge request: dotnet version

:clipboard: **Description**

I'd like to see a badge displaying the dotnet version the repository or nuget package is using.


:link: **Data**

The GitHub API does not give out this information, but nuget.org does, which means at least the second of the two example badges above should be possible.

In the API there is a dependency group for each target framework (which is what we are looking for; e.g. `net5.0`). If i understood the documentation from Microsoft correctly you'll probably end up [here](https://docs.microsoft.com/en-us/nuget/api/registration-base-url-resource#package-dependency-group).

:microphone: **Motivation**

When developing dotnet libraries we can choose several versions to compile against. It would be nice to show off in our repositories, that we support the newest dotnet framework version (currently `net5.0`; soon to be `net6.0`).

Start by reading the NuGet registration API documentation linked in the issue, especially the package dependency-group data, and inspect how Shields currently defines badge endpoints and data sources. Done means the NuGet-backed badge can expose a package's target framework, including the net5.0-style value described in the request.

Written by the indexing model from the issue text.