Deprecate IHttpAuthenticationFeature.IAuthenticationHandler

[davidfowl]

We deprecated the property on HttpContext but not on the feature interface

HttpAbstractions/src/Microsoft.AspNetCore.Http.Features/Authentication/IHttpAuthenticationFeature.cs

Line 12 in c06ed8c

IAuthenticationHandler Handler { get; set; }

/cc @Tratcher @HaoK

[Tratcher]

We're still using .User

[Tratcher]

Oh, I see. Nobody uses the feature interfaces directly

[davidfowl]

So we should just outright delete the property then.

[Tratcher]

There was some theory about not killing 1.1 middleware, but I don't know how realistic that is.