Bump error_prone_core from 2.6.0 to 2.7.1

Bumps [error_prone_core](https://github.com/google/error-prone) from 2.6.0 to 2.7.1.
- [Release notes](https://github.com/google/error-prone/releases)
- [Commits](google/error-prone@v2.6.0...v2.7.1)

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

Author: dependabot[bot]

pom.xml

@@ -46,7 +46,7 @@
     <javaVersion>7</javaVersion>
     <redirectTestOutputToFile>true</redirectTestOutputToFile>
     <project.build.outputTimestamp>2020-08-24T00:30:49Z</project.build.outputTimestamp>
-    <errorprone.version>2.6.0</errorprone.version>
+    <errorprone.version>2.7.1</errorprone.version>
   </properties>
 
   <dependencyManagement>