chore(deps): update dependency com.google.protobuf:protobuf-java to v3.15.8 (#1761)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| com.google.protobuf:protobuf-java | `3.15.7` -> `3.15.8` | [![age](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java/3.15.8/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java/3.15.8/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java/3.15.8/compatibility-slim/3.15.7)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-java/3.15.8/confidence-slim/3.15.7)](https://docs.renovatebot.com/merge-confidence/) |

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pom.xml

@@ -486,7 +486,7 @@
     <project.oauth.version>1.31.4</project.oauth.version>
     <project.jsr305.version>3.0.2</project.jsr305.version>
     <project.gson.version>2.8.6</project.gson.version>
-    <project.protobuf-java.version>3.15.7</project.protobuf-java.version>
+    <project.protobuf-java.version>3.15.8</project.protobuf-java.version>
     <project.guava.version>30.1.1-android</project.guava.version>
     <project.appengine.version>1.9.84</project.appengine.version>
     <project.xpp3.version>1.1.4c</project.xpp3.version>