Bump dubbo from 2.7.12 to 2.7.21 (#4)

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
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

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>