Disable importers by default in new self-manages instances

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/118970



Merged-by: drew stachon <730684-drew@users.noreply.gitlab.com>
Approved-by: Andrew Fontaine <afontaine@gitlab.com>
Approved-by: Marcos Rocha <mrocha@gitlab.com>
Approved-by: Minahil Nichols <minahilnichols@gitlab.com>
Approved-by: drew stachon <730684-drew@users.noreply.gitlab.com>
Reviewed-by: Andrew Fontaine <afontaine@gitlab.com>
Reviewed-by: Rodrigo Tomonari <rtomonari@gitlab.com>
Co-authored-by: Rodrigo Tomonari <rtomonari@gitlab.com>