Vulnerability UUID Migration Phase 5: Drop old uuid column and rename new_uuid
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## Problem
After GitLab has fully transitioned to using `new_uuid`, we can clean up by dropping the old `uuid` column and renaming `new_uuid` to `uuid`.
## Implementation
1. Verify all code paths are using `new_uuid` (Phase 4 complete)
2. Drop the old `uuid` column from `vulnerabilities`
3. Rename `new_uuid` column to `uuid`
4. Update any remaining references in code
5. Clean up feature flags from Phase 4
## Acceptance Criteria
- [ ] Old `uuid` column dropped
- [ ] `new_uuid` renamed to `uuid`
- [ ] All code references updated
- [ ] Feature flags cleaned up
- [ ] No regressions
## Dependencies
- Blocked by Phase 4 (transition must be complete and stable)
## Related
- Part of Vulnerability UUID migration series
- #587624 - Phase 1: Start populating new_uuid
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