Remove Hosting.Internal APIs

[Tratcher]

#11312 Everything became internal except IStartupConfigureContainerFilter and IStartupConfigureServicesFilter which became public but obsolete. These are only used in testing infrastructure, and only for WebHost.

I did need to clean up a mess where the Internal HostingApplication.Context had leaked into TestHost.

[johnnyreilly]

When the IStartupConfigureContainerFilter is actually removed then this will create issues for Autofac users relying on this workaround: https://blog.johnnyreilly.com/2020/10/02/autofac-6-integration-tests-and-generic-hosting/

This is to do with ConfigureTestContainer not working with GenericHost - context here: #14907

[ghost]

Hi @johnnyreilly. It looks like you just commented on a closed PR. The team will most probably miss it. If you'd like to bring something important up to their attention, consider filing a new issue and add enough details to build context.