Drop support for netstandard2.0 #3396
Description
The codebase is getting increasingly complicated (e.g. see #3283), and the lack of APIs available to consume with netstandard2.0 for the sake of supporting ASP.NET Core 2.1/2.3 for .NET Framework is causing a lot of additional complication we could do without (e.g. #if NET directives).
There doesn't appear to be any evidence that people are still actively using Swashbuckle.AspNetCore on .NET Framework and have a need for new features.
We don't even test for this scenario end-to-end anywhere, so there's a good chance it doesn't work anyway.
To make the project sustainable, we should consider dropping the netstandard2.0 targets and instead only target in-support versions of ASP.NET Core instead. For example, today that would be net8.0 and net9.0, with net10.0 in November 2025.
This would reduce the complexity of the code and make things more sustainable to maintain.
Support can be dropped in a major version ahead of the major version for .NET 10 support.