build(deps): bump protobuf from 5.29.5 to 5.29.6 in /backend

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 5.29.5 to 5.29.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-version: 5.29.6
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]