Bump com.thoughtworks.xstream:xstream from 1.4.20 to 1.4.21 (#1444)

Bumps [com.thoughtworks.xstream:xstream](https://github.com/x-stream/xstream) from 1.4.20 to 1.4.21.
- [Release notes](https://github.com/x-stream/xstream/releases)
- [Commits](https://github.com/x-stream/xstream/commits)

---
updated-dependencies:
- dependency-name: com.thoughtworks.xstream:xstream
  dependency-type: direct:production
...

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pom.xml

@@ -682,7 +682,7 @@
                                 <dependency>
                                     <groupId>com.thoughtworks.xstream</groupId>
                                     <artifactId>xstream</artifactId>
-                                    <version>1.4.20</version>
+                                    <version>1.4.21</version>
                                 </dependency>
                             </dependencies>
                         </plugin>