gwtp-basic-archetype should use the new tbroyer GWT maven plugin instead of the legacy Mojo's Maven Plugin for GWT

As mentioned on the home page of Mojo's GWT maven plugin (https://gwt-maven-plugin.github.io/gwt-maven-plugin/index.html), this plugin is now a legacy one, and the tbroyer GWT maven plugin is recommended to be used for new projects (https://github.com/tbroyer/gwt-maven-plugin).

The gwtp-basic archetype should use the new plugin instead of using the legacy one.