Update "Simple authorization" to 3.1

The code samples here are still valid in 3.0, but I think it would be helpful to introduce basic usage of endpoints and `RequireAuthorization` here.

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#### Document Details

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* ID: 4c3930e5-8a4f-8bdf-7167-9aed8afc4253
* Version Independent ID: cea5942d-fb01-f5bf-3b63-25873d5b79cf
* Content: [Simple authorization in ASP.NET Core](https://docs.microsoft.com/en-us/aspnet/core/security/authorization/simple?view=aspnetcore-3.1)
* Content Source: [aspnetcore/security/authorization/simple.md](https://github.com/aspnet/AspNetCore.Docs/blob/master/aspnetcore/security/authorization/simple.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-security**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**