Bump com.google.errorprone:error_prone_core from 2.20.0 to 2.21.0 (#196)

Bumps [com.google.errorprone:error_prone_core](https://github.com/google/error-prone) from 2.20.0 to 2.21.0.
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.20.0...v2.21.0)

---
updated-dependencies:
- dependency-name: com.google.errorprone:error_prone_core
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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pom.xml

@@ -381,7 +381,7 @@
                         <path>
                             <groupId>com.google.errorprone</groupId>
                             <artifactId>error_prone_core</artifactId>
-                            <version>2.20.0</version>
+                            <version>2.21.0</version>
                         </path>
                     </annotationProcessorPaths>
                 </configuration>