Bump dubbo from 2.7.18 to 2.7.21 in /test_hessian (#355)

Bumps [dubbo](https://github.com/apache/dubbo) from 2.7.18 to 2.7.21.
- [Release notes](https://github.com/apache/dubbo/releases)
- [Changelog](https://github.com/apache/dubbo/blob/3.2/CHANGES.md)
- [Commits](apache/dubbo@dubbo-2.7.18...dubbo-2.7.21)

---
updated-dependencies:
- dependency-name: org.apache.dubbo:dubbo
  dependency-type: direct:production
...

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test_hessian/pom.xml

@@ -45,7 +45,7 @@
         <dependency>
             <groupId>org.apache.dubbo</groupId>
             <artifactId>dubbo</artifactId>
-            <version>2.7.18</version>
+            <version>2.7.21</version>
         </dependency>
         <dependency>
             <groupId>com.alibaba</groupId>