Bump dubbo from 2.7.12 to 2.7.21

Bumps [dubbo](https://github.com/apache/dubbo) from 2.7.12 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-2.7.12...dubbo-2.7.21)

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

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

pom.xml

@@ -28,7 +28,7 @@
     <properties>
         <source.level>1.8</source.level>
         <target.level>1.8</target.level>
-        <dubbo.version>2.7.12</dubbo.version>
+        <dubbo.version>2.7.21</dubbo.version>
         <junit.version>4.12</junit.version>
         <spring.version>5.3.8</spring.version>
         <docker-maven-plugin.version>0.30.0</docker-maven-plugin.version>