deps: update dependency com.google.protobuf:protobuf-bom to v3.21.0 (#369)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.protobuf:protobuf-bom](https://developers.google.com/protocol-buffers/) ([source](https://togithub.com/protocolbuffers/protobuf)) | `3.20.1` -> `3.21.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.21.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.21.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.21.0/compatibility-slim/3.20.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.21.0/confidence-slim/3.20.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>protocolbuffers/protobuf</summary>

### [`v3.21.0`](https://togithub.com/protocolbuffers/protobuf/compare/v3.20.1...v3.21.0)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.20.1...v3.21.0)

</details>

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Author: renovate-bot

pom.xml

@@ -63,7 +63,7 @@
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <github.global.server>github</github.global.server>
     <site.installationModule>google-iam-parent</site.installationModule>
-    <protobuf.version>3.20.1</protobuf.version>
+    <protobuf.version>3.21.0</protobuf.version>
     <junit.version>4.13.2</junit.version>
     <grpc.version>1.46.0</grpc.version>
     <guava.version>31.0.1-android</guava.version>