Revert obsoleting CookieAuthenticationOptions.ExpireTimeSpan

[natemcmaster]

CookieAuthenticationOptions.Cookie.Expiration is conceptually different from CookieAuthenticationOptions.ExpireTimeSpan.

Un-oboleting the property and updating xmldocs.

[dnfclas]

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[HaoK]

Changes look fine, but are we missing some test coverage since shouldn't a test have to be updated? :) They probably have always been missing, but maybe we should take this opportunity to add 1-2 basic tests that actually enforce the behavior we expect?

[natemcmaster]

A bunch tests in CookieTests.cs were updated. Which behavior should we test for that isn't already covered?

[HaoK]

I mean the Cookie.Expires property I guess, or is that the one we basically ignore?

[natemcmaster]

Yeah, that's the one we currently ignore. I left this unchanged: https://github.com/aspnet/Security/blob/dev/src/Microsoft.AspNetCore.Authentication.Cookies/CookieAuthenticationHandler.cs#L180-L181

[HaoK]

Okay, worth adding a single test to capture that we ignore CookieBuilder.Expires so we don't regress this accidentally in the future?

[natemcmaster]

Added a test.

[natemcmaster]

Btw, let's use https://github.com/aspnet/Security/issues/1293 to clarify what CookieBuilder.Expiration should mean. We can address this after 2.0.0. For now, we'll just ignore its value.