Add redirects to removed Authentication pages

What does this MR do and why?

• Sets remove_date and redirect_to for two removed groupauthentication pages
• Deletes links from https://docs.gitlab.com/ee/update/deprecations.html to the removed pages

Related guidance

• https://docs.gitlab.com/ee/development/documentation/styleguide/deprecations_and_removals.html#remove-a-page

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