Bumping to frontend-m-p:1.6 for consistency

This has been bumped for instance in Blue Ocean already quite some time
ago. Also done in plugin-pom more recently.

war/pom.xml

@@ -604,7 +604,7 @@ THE SOFTWARE.
           <plugin>
             <groupId>com.github.eirslett</groupId>
             <artifactId>frontend-maven-plugin</artifactId>
-            <version>1.4</version>
+            <version>1.6</version>
             <executions>
 
               <execution>