Merge pull request apache#721 from apache/WW-5329-xstream

[WW-5329] Upgrades xstream to version 1.4.20

Author: lukaszlenart

pom.xml

@@ -1057,7 +1057,7 @@
             <dependency>
                 <groupId>com.thoughtworks.xstream</groupId>
                 <artifactId>xstream</artifactId>
-                <version>1.4.19</version>
+                <version>1.4.20</version>
                 <exclusions>
                     <exclusion>
                         <groupId>io.github.x-stream</groupId>