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

[davidfowl]

This is the discussion item for aspnet/Announcements#337

As part of #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.

[HaoK]

We have also removed all of the old 1.x authentication classes and namespaces as well:

Microsoft.AspNetCore.Http.Authentication and most of Microsoft.AspNetCore.Http.Features.Authentication

[HaoK]

The UseIdentity/UseGoogle/UseFacebook/UseTwitter/etc obsolete 1.x methods have all been removed as well in #6965

[aspnet-hello]

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