Pubternal APIs have been removed

[davidfowl]

In order to better maintain the public API surface of ASP.NET Core, we've moved a large majority of the types in the *.Internal namespace to be truly internal. Members in these namespaces were always in a gray area. They were never meant to be supported as public APIs and could break in minor releases (and often did). We've moved away from pubternal for a large majority of APIs (they are still a few edge cases) an as a result, code that used to depend on these APIs will be broken when moving to 3.0.

See these issues for more details:

• [Discussion] Making "pubinternal" types in MVC internal dotnet/aspnetcore#4932
• Remaining pubternal APIs in the shared framework dotnet/aspnetcore#11312.

[This announcement has been migrated to: dotnet/docs#14708]