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
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
diff --git a/johnzon-maven-plugin/pom.xml b/johnzon-maven-plugin/pom.xml
index 2b28f36..164012d 100644
--- a/johnzon-maven-plugin/pom.xml
+++ b/johnzon-maven-plugin/pom.xml
@@ -31,7 +31,7 @@
   <packaging>maven-plugin</packaging>
 
   <properties>
-    <maven.version>3.6.0</maven.version>
+    <maven.version>3.8.1</maven.version>
   </properties>