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]>

Author: dependabot[bot]

pom.xml

@@ -221,7 +221,7 @@
 								<dependency>
 									<groupId>com.thoughtworks.xstream</groupId>
 									<artifactId>xstream</artifactId>
-									<version>1.4.15</version>
+									<version>1.4.21</version>
 								</dependency>
 							</dependencies>
 						</plugin>