[Detection Engine][9.0] Deprecate legacy siem signals migration endpoints

[yctercero]

Summary

We propose removing the siem signals migration endpoints from Kibana in 10.0. These include the following routes:

POST /api/detection_engine/signals/migrations
DELETE /api/detection_engine/signals/migrations
POST /api/detection_engine/signals/finalize_migrations
GET /api/detection_engine/signals/migration_status

To prepare for this we need to:

• [ ] Deprecate POST /api/detection_engine/signals/migrations
• [ ] Deprecate DELETE /api/detection_engine/signals/migrations
• [ ] Deprecate POST /api/detection_engine/signals/finalize_migrations
• [ ] Deprecate GET /api/detection_engine/signals/migration_status
• [ ] Add telemetry to determine number of unmigrated .siem-signals indices
• [ ] Add check in upgrade assistant that identifies legacy siem signals and prompts user to utilize our APIs to migrate

### Docs checklist
- [ ] Create docs ticket to update endpoints as deprecated
- [ ] Create docs ticket to review upgrade details and add in requirement of migrating indices for 9.0

### QA checklist
- [ ] Create test plan for automated FTRs
- [ ] List manual testing that will be required and not covered by FTRs

[elasticmachine]

Pinging @elastic/security-detection-engine (Team:Detection Engine)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)