Brought up by `@bala.kumar` in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/218189, we would be better off to use a separate feature flag for using `undetected_since` in the ES query for the vulnerabilities over time chart. > turning the current feature flag on and off can leave backend systems out of sync, especially if vulnerabilities are detected again after being marked as no longer detected. If that happens, fixing the data later may require manual cleanup or additional migrations, which increases operational risk during rollout. > To reduce this risk, one idea @bala.kumar suggested is to separate data updates from user-visible behavior by using two feature flags: > * One flag controls whether we write and sync data changes. > * A second flag controls whether those changes are used in queries and UI behavior. Also see https://gitlab.com/groups/gitlab-org/-/epics/19780#note_3017336824 ### Implementation details 1. Introduce new WIP feature flag: `new_security_dashboard_over_time_chart_undetected_since` 2. Use instead of current feature flag in `ee/lib/search/advanced_finders/security/vulnerability/count_over_time_finder.rb` 3. Update spec accordingly