chore(deps): bump grpcio from 1.76.0 to 1.78.1 in /backend/python/vllm (#8635)

Bumps [grpcio](https://github.com/grpc/grpc) from 1.76.0 to 1.78.1.
- [Release notes](https://github.com/grpc/grpc/releases)
- [Commits](https://github.com/grpc/grpc/compare/v1.76.0...v1.78.1)

---
updated-dependencies:
- dependency-name: grpcio
  dependency-version: 1.78.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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