fix: Upgrade tsdown in create-turbo to resolve valibot ReDoS vulnerability (#11702)
## Summary
- Upgrades `tsdown` from `0.9.3` to `0.20.3` in `packages/create-turbo`
- This removes the transitive `valibot` dependency (`tsdown` →
`rolldown` → `valibot <1.2.0`) which had a ReDoS vulnerability
- `tsdown` is a devDependency used only for building, so the major
version bump has no runtime impact
Resolves TURBO-5234