Bump protobuf from 4.21.5 to 4.21.6 in /test (#323)

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 4.21.5 to 4.21.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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test/requirements.ci.txt

@@ -3,4 +3,4 @@ requests==2.28.1
 websocket-client==1.4.1
 urllib3==1.26.12
 grpcio==1.48.1
-protobuf==4.21.5
+protobuf==4.21.6