deps: update dependency protobuf to v4 (#25)

[![Mend Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

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/) |

---

### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Renovate will not automatically rebase this PR, because other commits have been found.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, click this checkbox. ⚠ **Warning**: custom changes will be lost.

---

This PR has been generated by [Mend Renovate](https://www.mend.io/free-developer-tools/renovate/). View repository job log [here](https://app.renovatebot.com/dashboard#github/googleapis/java-apikeys).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzMi4yMDguMSIsInVwZGF0ZWRJblZlciI6IjMyLjIwOC4xIn0=-->