Issue an error when using the source generators with older versions of C#

[papafe]

Nullability annotations have been introduced in C# 8.0, so they won't be supported in earlier versions.
We need to issue an error when developers use older versions of C#, otherwise they will have compilation errors when using the source generator

[carbonete]

Hi @papafe i have this problem in my Xamarin Forms app. This use .NetStandard 2.0 with C#7 , change to C# is not dificult but only need change .csproj to insert 8.0. But after this i receive errors
Error CS8400 Feature 'not pattern' is not available in C# 8.0. Please use language version 9.0 or greater.

I think deprecation of old and adopt only Interfaces can only be occurs more to end 2024.
The official support for Xamarin.Forms will end on May 1, 2024

[nirinchev]

Looks like we only use is not once in the generated class - we should easily be able to avoid this.