Bump com.thoughtworks.xstream:xstream from 1.4.15 to 1.4.21

Bumps [com.thoughtworks.xstream:xstream](https://github.com/x-stream/xstream) from 1.4.15 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
...

Signed-off-by: dependabot[bot] <[email protected]>

pom.xml

@@ -349,7 +349,7 @@
           <dependency>
             <groupId>com.thoughtworks.xstream</groupId>
             <artifactId>xstream</artifactId>
-            <version>1.4.15</version> <!-- apparently this needs to be exactly this version -->
+            <version>1.4.21</version> <!-- apparently this needs to be exactly this version -->
           </dependency>
         </dependencies>
       </plugin>