Bump error_prone_core from 2.6.0 to 2.8.1 (#146)

Bumps [error_prone_core](https://github.com/google/error-prone) from 2.6.0 to 2.8.1.
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.6.0...v2.8.1)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

pom.xml

@@ -48,7 +48,7 @@
     <project.build.outputTimestamp>2020-08-24T00:30:49Z</project.build.outputTimestamp>
     <junit.version>4.13.2</junit.version>
     <aspectj.version>1.9.7.M3</aspectj.version>
-    <errorprone.version>2.6.0</errorprone.version>
+    <errorprone.version>2.8.1</errorprone.version>
   </properties>
 
   <dependencyManagement>