Merge branch 'nd/fix-epix-hierarchy-depth-check' into 'master'

Fix depth check when adding existing epics

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/111442



Merged-by: James Fargher <proglottis@gmail.com>
Approved-by: James Fargher <proglottis@gmail.com>
Approved-by: Ian Anderson <ianderson@gitlab.com>
Co-authored-by: Nicolas Dular <ndular@gitlab.com>