ObjectPoolProvider removed from the default set of dependencies added in the WebHostBuilder

[davidfowl]

As part of making ASP.NET Core more pay for play, we moved the ObjectPoolProvider out of the main set of dependencies to the specific components that were using them.

If you were relying on the ObjectPoolProvider being available by default, you should add them to your dependencies via the IServiceCollection.

For discussion, please use dotnet/aspnetcore#5944

[This announcement has been migrated to: dotnet/docs#14736]