Merge branch '519217-rename-custom-models-directory-and-componentry' into 'master'

Rename custom_models directory to duo_self_hosted

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181744



Merged-by: Savas Vedova <svedova@gitlab.com>
Approved-by: Lukas Wanko <lwanko@gitlab.com>
Approved-by: Savas Vedova <svedova@gitlab.com>
Reviewed-by: Lukas Wanko <lwanko@gitlab.com>
Co-authored-by: Julie Huang <julhuang@gitlab.com>