fix: Upgrade git2 to fix RUSTSEC-2026-0008 (#11729)
## Summary
- Upgrades `git2` from 0.20.0 to 0.20.4 to fix RUSTSEC-2026-0008
(potential undefined behavior when dereferencing Buf struct)
- No source code changes needed — the upgrade is API-compatible
Resolves TURBO-5249
