Bump com.thoughtworks.xstream:xstream in /helix-rest (#2996)

Bumps [com.thoughtworks.xstream:xstream](https://github.com/x-stream/xstream) from 1.4.19 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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helix-rest/pom.xml

@@ -121,7 +121,7 @@
     <dependency>
       <groupId>com.thoughtworks.xstream</groupId>
       <artifactId>xstream</artifactId>
-      <version>1.4.19</version>
+      <version>1.4.21</version>
     </dependency>
     <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>