Project is not compatible with OSX

> Please provide us with the following information:
> ---------------------------------------------------------------

### This issue is for a: (mark with an `x`)
```
- [X] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> Run `dotnet run` on a Mac

### Any log messages given by the failure
>
```
✗ dotnet run
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '2.1.0' was not found.
- The following frameworks were found:
3.1.6 at [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
5.0.0 at [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
- https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=2.1.0&arch=x64&rid=osx.10.15-x64
```

However, .NET core 2.1 Runtime is not available on OSX

### Expected/desired behavior
>

### OS and Version?
> OSX 10.15.7

### Versions
>

### Mention any other details that might be useful

> ---------------------------------------------------------------
> Thanks! We'll be in touch soon.

Beginne damit, `dotnet run` unter OSX 10.15.7 auszuführen und die Projektkonfiguration zu prüfen, die Microsoft.NETCore.App 2.1 auswählt. Bestätige, welche unterstützte Runtime oder welches unterstützte Target für macOS verfügbar ist. Als erledigt gilt die Aufgabe, wenn die Anwendung auf dem angegebenen System erfolgreich startet, ohne die nicht verfügbare .NET Core 2.1-Runtime zu benötigen.

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