[pom] Require maven 3.6.3 per maven support

Maven has officially flagged all versions 3.6.3 and before as end of life.  All plugins are encouraged to require at least 3.6.3 going forwards.  Many core plugins have already started requiring this.

license-maven-plugin/pom.xml

@@ -33,6 +33,10 @@
   <inceptionYear>2008</inceptionYear>
   <url>https://oss.carbou.me/license-maven-plugin</url>
 
+  <prerequisites>
+    <maven>3.6.3</maven>
+  </prerequisites>
+
   <scm>
     <connection>scm:git:https://github.com/mathieucarbou/license-maven-plugin.git</connection>
     <developerConnection>scm:git:git@github.com:mycila/license-maven-plugin.git</developerConnection>