Change wording for self-managed customers on top-level groups

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/117225



Merged-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Kamil Trzciński <ayufan@ayufan.eu>
Approved-by: Christina Lohr <clohr@gitlab.com>
Co-authored-by: Alana Bellucci <abellucci@gitlab.com>
Co-authored-by: Sander Brienen <sbrienen@gitlab.com>