bump xstream to 1.4.20

Author: vladak

suggester/pom.xml

@@ -114,7 +114,7 @@ Portions Copyright (c) 2020, Chris Fraire <[email protected]>.
         <dependency>
             <groupId>com.thoughtworks.xstream</groupId>
             <artifactId>xstream</artifactId>
-            <version>1.4.19</version>
+            <version>1.4.20</version>
         </dependency>
 
         <dependency>