Update self-managed subscription expiration section

Our expiration policy seems out of date. When searching the docs for "expiration" the page that shows us is: https://docs.gitlab.com/ee/user/admin_area/license.html#what-happens-when-your-license-expires

I believe this should have information about both SaaS and self-managed subscriptions.

I've just opened an issue about this same topic: #325346 (closed)

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