Upgrade to error-prone 2.6.0

Release notes:

https://github.com/google/error-prone/releases/tag/v2.6.0

pom.xml

@@ -115,7 +115,7 @@
             <path>
               <groupId>com.google.errorprone</groupId>
               <artifactId>error_prone_core</artifactId>
-              <version>2.5.1</version>
+              <version>2.6.0</version>
             </path>
           </annotationProcessorPaths>
         </configuration>