Removed deprecated Authentication property from HttpContext (including all of 1.x auth stack)

[davidfowl]

As part of dotnet/aspnetcore#6504 we've removed the deprecated Authentication property. The Authentication property has been deprecated since 2.0 and a migration guide was published to migrate code using this deprecated property to the new replacement APIs.

Please discussion this on dotnet/aspnetcore#6533

[This announcement has been migrated to: dotnet/docs#14732]

[HaoK]

Note: we have also removed all of the remaining unused classes/apis related to the old 1.x authentication stack in dotnet/aspnetcore@d7a7c65

This includes:

// Classes
Microsoft.AspNetCore.Http.Authentication.AuthenticationManager
Microsoft.AspNetCore.Http.Authentication.AuthenticateInfo
Microsoft.AspNetCore.Http.Authentication.AuthenticationProperties
Microsoft.AspNetCore.Http.Features.Authentication.AuthenticateContext
Microsoft.AspNetCore.Http.Features.Authentication.ChallengeContext
Microsoft.AspNetCore.Http.Features.Authentication.ChallengeBehavior
Microsoft.AspNetCore.Http.Features.Authentication.DescribeSchemesContext
Microsoft.AspNetCore.Http.Features.Authentication.IAuthenticationHandler
Microsoft.AspNetCore.Http.Features.Authentication.SignInContext
Microsoft.AspNetCore.Http.Features.Authentication.SignOutContext

// Property
Microsoft.AspNetCore.Http.Features.Authentication.IHttpAuthenticationFeature.Handler property