Add deprecations to CAS and Crowd global navigation items

What does this MR do and why?

Companion MR to: gitlab!95017 (merged).

Adds (deprecated) to some entries on the global nav, per: https://docs.gitlab.com/ee/development/documentation/versions.html#deprecate-a-page-or-topic.

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.