deps: update dependency com.google.protobuf:protobuf-java-util to v3.21.0 (googleapis#779)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.protobuf:protobuf-java-util](https://developers.google.com/protocol-buffers/) ([source](https://github.com/protocolbuffers/protobuf)) | `3.20.1` -> `3.21.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java-util/3.21.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java-util/3.21.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java-util/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-java-util/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://github.com/protocolbuffers/protobuf/compare/v3.20.1...v3.21.0)

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

</details>

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Author: renovate-bot

java-secretmanager/samples/install-without-bom/pom.xml

@@ -36,7 +36,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java-util</artifactId>
-      <version>3.20.1</version>
+      <version>3.21.0</version>
     </dependency>
     <dependency>
       <groupId>junit</groupId>

java-secretmanager/samples/snapshot/pom.xml

@@ -34,7 +34,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java-util</artifactId>
-      <version>3.20.1</version>
+      <version>3.21.0</version>
     </dependency>
 
     <dependency>

java-secretmanager/samples/snippets/pom.xml

@@ -46,7 +46,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java-util</artifactId>
-      <version>3.20.1</version>
+      <version>3.21.0</version>
     </dependency>
     <dependency>
       <groupId>junit</groupId>