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.

Inizia eseguendo `dotnet run` su OSX 10.15.7 e ispeziona la configurazione del progetto che seleziona Microsoft.NETCore.App 2.1. Conferma quale runtime o target supportato è disponibile per macOS. Il lavoro è completato quando l’applicazione si avvia correttamente sul sistema indicato senza richiedere il runtime .NET Core 2.1 non disponibile.

Scritto dal modello di indicizzazione a partire dal testo della issue.