Update code to use WebApplication.CreateBuilder

[lord-executor]

With https://github.com/aspnet/Announcements/issues/526, Microsoft has deprecated the WebHostBuilder, IWebHost, and WebHost in favor of the newer HostBuilder and WebApplication.CreateBuilder. Since the configuration model of Ninject.Web.AspNetCore is currently very directly tied to the capabilities of the WebHostBuilder, it seems likely that updating to the new APIs will incur some breaking changes and as such requires some careful consideration. Even if breaking changes aren't actually needed it might still be a good idea to change some of the configuration API surface to make it easier to apply customizations.