Bump com.thoughtworks.xstream:xstream in /geowebcache

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]

geowebcache/pom.xml

@@ -56,7 +56,7 @@
     <jaiext.version>1.1.27</jaiext.version>
     <spring.version>5.3.39</spring.version>
     <spring.security.version>5.7.13</spring.security.version>
-    <xstream.version>1.4.20</xstream.version>
+    <xstream.version>1.4.21</xstream.version>
     <commons-io.version>2.16.1</commons-io.version>
     <commons-dbcp.version>1.4</commons-dbcp.version>
     <commons-collections.version>4.4</commons-collections.version>