Update log4j (testing only)

Author: jdhoek

pom.xml

@@ -47,7 +47,7 @@
     <javax.annotation.version>1.3.2</javax.annotation.version>
 
     <!-- For testing only. -->
-    <log4j.version>2.15.0</log4j.version>
+    <log4j.version>2.17.1</log4j.version>
     <hamcrest.optional>1.0</hamcrest.optional>
   </properties>