Update protobuf requirement in /tests

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...v5.26.0)

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

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

tests/constraints.txt

@@ -16,7 +16,7 @@ test-generator==0.1.2
 requests>=2.25.1
 opencv-python>=4.5
 paddlepaddle==2.5.0
-protobuf>=3.18.1,<4.0.0
+protobuf>=3.18.1,<6.0.0
 py>=1.9.0
 pytest>=5.0,<7.5
 pytest-dependency==0.5.1