Revert "Bump com.github.siom79.japicmp:japicmp-maven-plugin (#5077)"

This reverts commit a4edcb9.

Author: dagnir

pom.xml

@@ -156,7 +156,7 @@
         <exec-maven-plugin.version>1.6.0</exec-maven-plugin.version>
         <maven-deploy-plugin.version>2.8.2</maven-deploy-plugin.version>
         <build-helper-maven-plugin.version>3.3.0</build-helper-maven-plugin.version>
-        <japicmp-maven-plugin.version>0.20.0</japicmp-maven-plugin.version>
+        <japicmp-maven-plugin.version>0.15.6</japicmp-maven-plugin.version>
         <versions-maven-plugin.version>2.13.0</versions-maven-plugin.version>
         <maven-archetype-plugin.version>3.2.1</maven-archetype-plugin.version>
         <!-- Whenever we update maven-wrapper-plugin version, we need to run mvn wrapper:wrapper to update the Maven Wrapper files(mvnw, .maven and mvnw.cmd) -->