Bump com.thoughtworks.xstream:xstream in /suggester

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
...

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

Author: dependabot[bot]

suggester/pom.xml

@@ -121,7 +121,7 @@ Portions Copyright (c) 2020, Chris Fraire <[email protected]>.
         <dependency>
             <groupId>com.thoughtworks.xstream</groupId>
             <artifactId>xstream</artifactId>
-            <version>1.4.20</version>
+            <version>1.4.21</version>
         </dependency>
 
         <dependency>