deps: update dependency protobuf to v4 (#261)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [protobuf](https://developers.google.com/protocol-buffers/) | `==3.19.5` -> `==4.21.6` | [![age](https://badges.renovateapi.com/packages/pypi/protobuf/4.21.6/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/protobuf/4.21.6/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/protobuf/4.21.6/compatibility-slim/3.19.5)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/protobuf/4.21.6/confidence-slim/3.19.5)](https://docs.renovatebot.com/merge-confidence/) |

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