Update Error Prone to v2.20.0 (#490)

Manually update error-prone.

Signed-off-by: ascopes <[email protected]>

Author: ascopes

acceptance-tests/acceptance-tests-error-prone/pom.xml

@@ -50,7 +50,7 @@
       -DmvnArgLinePropagated=true
     </argLine>
 
-    <error-prone.version>2.18.0</error-prone.version>
+    <error-prone.version>2.20.0</error-prone.version>
   </properties>
 
   <dependencies>