Group Secret Rotation Background Job
## Feature
Automated rotation reminders for group secrets
## Scope
- Background worker for group secret rotation reminders
- Scan for secrets needing rotation reminders
- Send notifications to group owners/maintainers
- Update `last_reminder_at` and `next_reminder_at` timestamps
- Metrics and logging
## Deliverable
Automated rotation reminders for group secrets
## Dependencies
https://gitlab.com/gitlab-org/gitlab/-/issues/577344
## Reference
Similar to project secret rotation worker
Related to https://gitlab.com/groups/gitlab-org/-/epics/17904
issue