The new IHostEnvironment, IsDevelopment, IsProduction etc. extension methods are in the Microsoft.Extensions.Hosting namespace which may need to be added to your app.
Reference:
https://learn.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.0&tabs=visual-studio#migrate-startupconfigure
https://github.com/aspnet/AspNetCore/issues/7749