Merge branch '452467-vue3-migration-form_custom_header' into 'master'

Remove Vue set for vue 3 migration

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157387



Merged-by: Jannik Lehmann <jlehmann@gitlab.com>
Approved-by: Ammar Alakkad <aalakkad@gitlab.com>
Approved-by: Jannik Lehmann <jlehmann@gitlab.com>
Reviewed-by: Jannik Lehmann <jlehmann@gitlab.com>
Co-authored-by: Artur Fedorov <afedorov@gitlab.com>