Add REST API deprecation warning on Epics API docs

What does this MR do and why?

Add REST API deprecation warning on follow Epics API docs;

• https://docs.gitlab.com/ee/api/epic_issues.html
• https://docs.gitlab.com/ee/api/epic_links.html
• https://docs.gitlab.com/ee/api/linked_epics.html
• https://docs.gitlab.com/ee/api/discussions.html#epics

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#430080