Enhance Blazor 8.0 migration guidance

[guardrex]

Addresses #32138

@mkArtakMSFT ... For dotnet/aspnetcore#55678 (comment) and the task of adding migration guidance, it's not clear from the discussion what a user would be migrating from or the conditions that create a problem for our base coverage. The dev seems to be asking about a specific custom approach that they took and a solution/workaround that Halter is giving them. What is the starting point for the migration scenario? I assume the solution to that problem is to call Microsoft.AspNetCore.Builder.AuthorizationEndpointConventionBuilderExtensions.AllowAnonymous with a correctly-configured <AuthorizeRouteView> in the Routes component.

@danroth27 For ...

We changed our authentication guidance in .NET 8, but we didn't give detailed instructions on how to upgrade

Do you want migration guidance from Blazor Server with MS Identity Web to our BWA+OIDC non-BFF guidance? ... and noting that our BWA+OIDC guidance adopts global Interactive Auto rendering ... and noting further that we don't have BWA MS Identity Web guidance, only OIDC at this time. I mention that because the natural migration would be for MS Identity Web with Blazor Server to MS Identity Web with BWA; however, we don't have MS Identity Web BWA reference guidance. We'd be migrating to an app that we don't actually document. However again 😄, it's strange to migrate away from MS Identity Web to OIDC if MS Identity Web is what the dev was using.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/host-and-deploy/webassembly-deployment-layout.md	Deployment layout for ASP.NET Core hosted Blazor WebAssembly apps
aspnetcore/migration/70-80.md	[Visual Studio for Mac](https://review.learn.microsoft.com/en-us/aspnet/core/migration/70-80?branch=pr-en-us-33038)

[danroth27]

Do you want migration guidance from...

@guardrex We need to document how to upgrade existing .NET 7 Blazor apps that use auth to .NET 8. This includes both Blazor Server & Blazor WebAssembly apps that use ASP.NET Core Identity and the Microsoft Identity Platform (Entra). For Entra, @halter73 is planning to provide a sample in a few weeks. For apps that use ASP.NET Core Identity & Identity Server, I think we need guidance from @JeremyLikness.

[guardrex]

I'll change this to an "addresses" PR and prepare to merge it after I hear back from @mkArtakMSFT on the other item ☝️ because it sounds like some of the auth targets for migration are still in the works. Later, I'll need clarification on the exact starting points.

Examples of scenarios that we didn't cover in the Blazor node:

• Duende backend for Blazor WASM, which I suppose is the 7.0 starting point for the 8.0 Standalone with Identity (MapIdentityApi) article+sample target.
• Blazor Server+Duende, which I suppose is the 7.0 starting point for the 8.0 BWA+OIDC (non-BFF?) article+sample.
• Blazor Server+Entra, which is obviously the 7.0 starting point for 8.0 BWA+Entra. IIRC, the Azure folks were going to put up samples for BWA+Entra and BWA+B2C. I see you're saying now that Halter will create the target (at least for Entra), so I'll just need to start with a Blazor Server+Entra app.

I think that WASM+Entra/B2C doesn't require more than package and TFM changes, which are already covered by the existing migration guidance.

[guardrex]

I'll proceed with this in this form, and we can take up the other two items later on the issue and one or more new PRs.