Deprecate and remove Twitter OAuth 1.0

What does this MR do?

Due to low use, lack of support, and lack of a functional login option, we are deprecating Twitter OmniAuth.

• For self-managed GitLab, this is being deprecated in 16.3 and removed in 17.0.
• For SaaS, it has been agreed to do expedite the removal of this feature through a breaking change exception. Therefore it is being deprecated and removed in 16.3.

-This MR creates 2 separate deprecation notices, updates the deprecations and removals documentation, and adds deprecation content to the documentation.

Related issues

gitlab-com/Product#11417 (closed)

Author's checklist

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Reviewer's checklist

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