Bump protobuf from 4.21.4 to 4.21.6

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 4.21.4 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: indirect
...

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Author: dependabot[bot]