Update protobuf requirement in /tools

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/protobuf_release.bzl)
- [Commits](protocolbuffers/protobuf@v3.18.1...v5.29.3)

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

Signed-off-by: dependabot[bot] <[email protected]>

tools/constraints.txt

@@ -5,7 +5,7 @@
 h5py>=3.1.0,<3.11.0
 onnx>=1.8.1,<=1.17.0
 pytest>=5.0,<8.4
-protobuf>=3.18.1,<4.0.0
+protobuf>=3.18.1,<6.0.0
 defusedxml>=0.7.1
 requests>=2.25.1
 coverage>=4.4.2,<=7.0.5