chore(deps): update dependency com.google.cloud:libraries-bom to v21 (#247)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:libraries-bom](https://togithub.com/GoogleCloudPlatform/cloud-opensource-java) | `20.9.0` -> `21.0.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:libraries-bom/21.0.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:libraries-bom/21.0.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:libraries-bom/21.0.0/compatibility-slim/20.9.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:libraries-bom/21.0.0/confidence-slim/20.9.0)](https://docs.renovatebot.com/merge-confidence/) |

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Author: renovate-bot

privateca/pom.xml

@@ -40,7 +40,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>libraries-bom</artifactId>
-        <version>20.9.0</version>
+        <version>21.0.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>