Bump error_prone_core from 2.8.1 to 2.9.0

Bumps [error_prone_core](https://github.com/google/error-prone) from 2.8.1 to 2.9.0.
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.8.1...v2.9.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

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>2.8.1</errorprone.version>
+    <errorprone.version>2.9.0</errorprone.version>
   </properties>
 
   <dependencyManagement>