Merge branch 'msj-child-epics-tier' into 'master'

Clarify child epics availability

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/144310



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Co-authored-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>