Manually created error budget dashboards should not be deleted
As part of gitlab-org/gitlab#407764 (closed)
In a previous MR, the not_owned
error budget dashboard was added. However, when running the script update_stage_groups_dashboards.rb
the dashboard is being deleted given the fact of not_owned
not being a group de-facto.
This MR introduces a small change allowing to keep the not_owned dashboard.