Bump japicmp to 0.21.0 (netty#13995)

Motivation:

Bump japicmp to 0.21.0

Modification:

There was an issue in the newer version, but after reported now it's
fixed.
siom79/japicmp#393
siom79/japicmp#394

Result:

Bump japicmp to 0.21.0

---------

Co-authored-by: Norman Maurer <norman_maurer@apple.com>

pom.xml

@@ -1375,7 +1375,7 @@
       <plugin>
         <groupId>com.github.siom79.japicmp</groupId>
         <artifactId>japicmp-maven-plugin</artifactId>
-        <version>0.15.7</version>
+        <version>0.21.0</version>
         <configuration>
           <parameter>
             <ignoreMissingOldVersion>true</ignoreMissingOldVersion>