New doc: How to upgrade ASP.NET Core 2.0 to 2.1

[Rick-Anderson]

URL must be https://docs.microsoft.com/en-us/aspnet/core/migration/20-to-21/?view=aspnetcore-2.1

Title Migrate from ASP.NET Core 2.0 to 2.1
See #6069 ?
Update link in

uid: aspnetcore-2.1
# What's new in ASP.NET Core 2.1

See [Migrate from ASP.NET Core 2.0 to 2.1](xref:migration/index)

See https://www.stevejgordon.co.uk/upgrading-aspnet-core-2-application-aspnet-core-2-1-preview-1 and ask Steve Gordon to review.

[scottaddie]

@Rick-Anderson See the instructions here: https://blogs.msdn.microsoft.com/webdev/2018/05/07/asp-net-core-2-1-0-rc1-now-available/#migrating-an-asp-net-core-2-0-x-project-to-2-1-0-rc1

[Rick-Anderson]

All that's in CompatibilityVersion and #6069
I'll show the new code the templates generate.

[Rick-Anderson]

@natemcmaster can you give me the elevator pitch on why the templates went from BuildWebHost to CreateWebHostBuilder

I'd like to add that to this migration doc.

[Rick-Anderson]

@natemcmaster bonus if you can tell me why "AllowedHosts": "*" was added to appsettings.json.

[natemcmaster]

AllowedHosts was added due to dotnet/aspnetcore#2954. @Tratcher is the expert on this one.

WebHostBuilder changes were made to make it easier to write integration tests. @javiercn's can share more.

[Rick-Anderson]

@guardrex can you review what I say about CreateWebHostBuilder and the links I provide to your testing doc in PR #6612.

[Tratcher]

Nate is correct on AllowedHosts. 2.1 has a small new host filtering middleware included in CreateDefaultBuilder and setting a real host name in AllowedHosts activates it. The * is only a placeholder for discoverability.

[guardrex]

@Rick-Anderson Done https://github.com/aspnet/Docs/pull/6612/files#r191049387

[javiercn]

We needed to get a hold of the IWebHostBuilder for testing purposes, so we switched it to CreateWebHostBuilder