[MNGSITE-393] remove more pre Maven 3 content

[elharo]

@eolivelli

[pzygielo]

With this change, this

maven-jar-plugin/src/main/java/org/apache/maven/plugins/jar/AbstractJarMojo.java

Line 109 in fdac54f

* <a href="https://maven.apache.org/plugins/maven-jar-plugin/">Major Version Upgrade to version 3.0.0</a> for the

or moreover

maven-jar-plugin/src/main/java/org/apache/maven/plugins/jar/AbstractJarMojo.java

Lines 299 to 301 in fdac54f

	throw new MojoExecutionException( "You are using 'useDefaultManifestFile' which has been removed"
	+ " from the maven-jar-plugin. "
	+ "Please see the >>Major Version Upgrade to version 3.0.0<< on the plugin site." );

is hardly helpful 😞