Notice on obsoleted RazorEngine APIs in Razor

## Notice on obsoleted RazorEngine APIs in Razor

Types related to the `RazorEngine` type in Blazor have been obsoleted.

### Version introduced

6.0.0-preview1

### Old behavior

APIs were not obsolete.

### New behavior

APIs are obsoleted.

### Reason for change

The `RazorEngine` type has been deprecated in favor of the `RazorProjectEngine` type.

### Recommended action

Migrate source code from the `RazorEngine` type to the `RazorProjectEngine` type.

### Category

ASP.NET

### Affected APIs

- [Microsoft.AspNetCore.Mvc.Razor.Extensions.InjectDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.razor.extensions.injectdirective.register?view=aspnetcore-3.1)
- [Microsoft.AspNetCore.Mvc.Razor.Extensions.ModelDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.razor.extensions.namespacedirective.register?view=aspnetcore-2.2)
- [Microsoft.AspNetCore.Mvc.Razor.Extensions.PageDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.razor.extensions.namespacedirective.register?view=aspnetcore-2.2)
- [Microsoft.AspNetCore.Razor.Language.Extensions.FunctionsDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.razor.language.extensions.functionsdirective.register?view=aspnetcore-3.0)
- [Microsoft.AspNetCore.Razor.Language.Extensions.InheritsDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.razor.language.extensions.inheritsdirective.register?view=aspnetcore-3.0)
- [Microsoft.AspNetCore.Razor.Language.Extensions.SectionDirective.Register](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.razor.language.extensions.sectiondirective.register?view=aspnetcore-3.0)
- [Microsoft.AspNetCore.Razor.Language.IRazorEngineBuilder](https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.razor.language.irazorenginebuilder?view=aspnetcore-3.0)



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#### Issue metadata

* Issue type: breaking-change

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Notice on obsoleted RazorEngine APIs in Razor #446