Merge branch 'docs-update-epic-rest-api-pages-deprecation' into 'master'

Add REST API deprecation warning on Epics API docs

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/167363



Merged-by: Phillip Wells <pwells@gitlab.com>
Approved-by: Phillip Wells <pwells@gitlab.com>
Co-authored-by: Kushal Pandya <kushal@gitlab.com>