Bump org.apache.maven:maven-core in /johnzon-maven-plugin (#124)

Bumps [org.apache.maven:maven-core](https://github.com/apache/maven) from 3.6.0 to 3.8.1.
- [Release notes](https://github.com/apache/maven/releases)
- [Commits](https://github.com/apache/maven/compare/maven-3.6.0...maven-3.8.1)

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

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