Upgrade maven-plugin-tools to support Java 11 generated Mojo

diff --git a/pom.xml b/pom.xml
index fe7e82a..ae4025e 100644
--- a/pom.xml
+++ b/pom.xml
@@ -95,6 +95,7 @@
     <invoker.install.skip>${invoker.skip}</invoker.install.skip>
     <invoker.it.skip>${invoker.skip}</invoker.it.skip>
     <invoker.cloneClean>true</invoker.cloneClean>
+    <mavenPluginToolsVersion>3.6.0</mavenPluginToolsVersion><!-- required for Java 11 generated Mojo -->
     <checkstyle.violation.ignore>ParameterNumber,MethodLength</checkstyle.violation.ignore>
     <project.build.outputTimestamp>2020-09-26T20:10:30Z</project.build.outputTimestamp>
   </properties>