Obsolete legacy Blazor hosting models in favor of the unified Blazor Web App model

.NET 8 introduced the Blazor Web App model, which unified the Blazor Server and Blazor WebAssembly hosting model under a single Blazor app model. Blazor Web Apps support both server-side rendering (SSR) and client-side interactivity (WebAssembly), enabling flexible rendering scenarios and simplifying the hosting story. To reduce confusion around hosting choices, eliminate redundant APIs, and focus innovation and support on a single, modern approach, we propose to obsolete the legacy Blazor hosting models.

Related APIs and framework features to be considered for obsoletion:

- Legacy Blazor Server:
- `AddServerSideBlazor`
- `MapBlazorHub`
- *blazor.server.js*
- Legacy ASP.NET Core hosted Blazor WebAssembly:
- `UseBlazorFrameworkFiles`

Standalone Blazor WebAssembly apps will still be supported for static site scenarios.

## Considerations:

We should investigate simplifying migration to the Blazor Web App mode by provide guidance and tooling.

We will likely need to first address feature gaps in the Blazor Web App model that may prevent users from migrating. For example:

- Integration with MVC Views and Razor Pages via the component tag helper.
- Progressive Web App support