WI lists stage 3 - Group issues list
### Motivation
### Feature flags
`work_items_group_issues_list` instructs the group issues page to load the Work Items List app, rather than the legacy issues list app.
WARNING: `work_item_planning_view` will prevent you from visiting the proper issues list and you'll visit the consolidated list instead.
### Rollout Plan
* [x] Switch `work_items_group_issues_list` for internal actors (gitlab-org) - @ramistry 2025/09/24 3pm UTC / @mdangelo6 2025-09-26 5am UTC
* [ ] Monitoring plan (performance, errors), so our error budgets don’t drop - @mdangelo6
- Dashboard: https://log.gprd.gitlab.net/app/dashboards#/view/02af4d7b-59b5-4997-8024-489a9d8824b5?_g=h@2294574
* [x] Gradual rollout plan, for the `work_items_group_issues_list` feature flag
* [x] 10 % @vjain-gl - 2025-10-06
* [x] 25 % @vjain-gl - 2025-10-07
* [x] 50 % @vjain-gl - 2025-10-08
* [x] 100 % @vjain-gl - 2025-10-09
* [x] Default enable feature flag @vjain-gl - 2025-10-09
* [ ] Announcements - sync up with Amanda
* TBD
* [ ] Clean up %"18.6"
* [x] remove the feature flags
* [ ] remove the legacy code
epic