…ugin to v3.12.1 (googleapis#961)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [org.apache.maven.plugins:maven-site-plugin](https://maven.apache.org/plugins/) | `3.12.0` -> `3.12.1` | [![age](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-site-plugin/3.12.1/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-site-plugin/3.12.1/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-site-plugin/3.12.1/compatibility-slim/3.12.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/org.apache.maven.plugins:maven-site-plugin/3.12.1/confidence-slim/3.12.0)](https://docs.renovatebot.com/merge-confidence/) |

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