Bump japicmp-maven-plugin from 0.15.4 to 0.15.6

Bumps [japicmp-maven-plugin](https://github.com/siom79/japicmp) from 0.15.4 to 0.15.6.
- [Release notes](https://github.com/siom79/japicmp/releases)
- [Changelog](https://github.com/siom79/japicmp/blob/master/release.py)
- [Commits](siom79/japicmp@japicmp-base-0.15.4...japicmp-base-0.15.6)

---
updated-dependencies:
- dependency-name: com.github.siom79.japicmp:japicmp-maven-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -54,7 +54,7 @@
         <checkstyle.plugin.version>3.1.2</checkstyle.plugin.version>
         <checksyle.version>9.2.1</checksyle.version>
         <japicmp.skip>false</japicmp.skip>
-        <commons.japicmp.version>0.15.4</commons.japicmp.version>
+        <commons.japicmp.version>0.15.6</commons.japicmp.version>
         <commons.pmd.version>3.15.0</commons.pmd.version>
         <commons.spotbugs.version>4.5.3.0</commons.spotbugs.version>