Upgrade error_prone_core to 2.0.21

And plexus-compiler-javac-errorprone to 2.8.2

Change-Id: I5925a6511d8d7ed53922d969eef7320429190184

pom.xml

diff --git a/pom.xml b/pom.xml
index 89258d9..8737499 100644
--- a/pom.xml
+++ b/pom.xml
@@ -392,14 +392,14 @@
           <dependency>
             <groupId>org.codehaus.plexus</groupId>
             <artifactId>plexus-compiler-javac-errorprone</artifactId>
-            <version>2.8.1</version>
+            <version>2.8.2</version>
           </dependency>
           <!-- override plexus-compiler-javac-errorprone's dependency on
                Error Prone with the latest version -->
           <dependency>
             <groupId>com.google.errorprone</groupId>
             <artifactId>error_prone_core</artifactId>
-            <version>2.0.19</version>
+            <version>2.0.21</version>
           </dependency>
         </dependencies>
       </plugin>