Simplify groovydoc generation

[rolnico]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Quality

What is the current behavior?

Use build-helper-maven-plugin to attach groovydoc.

What is the new behavior (if this is a feature change)?
Use gmaven-plusin:groovydoc-jar to attach groovydoc.

Does this PR introduce a breaking change or deprecate an API?

• The Breaking Change or Deprecated label has been added
• The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

What changes might users need to make in their application due to this PR? (migration steps)

If one of your pom.xml contains a build-helper-maven-plugin execution in order to attach groovydoc with type javadoc:

• remove this plugin execution;
• add the property <groovydoc.attach>true</groovydoc.attach>.

For instance:

    <properties>
+        <groovydoc.attach>true</groovydoc.attach>
        <groovydoc.classifier>javadoc</groovydoc.classifier>
    </properties>
<!-- ... -->
    <build>
            <!-- ... -->
            <plugin>
                <groupId>org.codehaus.gmavenplus</groupId>
                <artifactId>gmavenplus-plugin</artifactId>
            </plugin>
-            <plugin>
-                <groupId>org.codehaus.mojo</groupId>
-                <artifactId>build-helper-maven-plugin</artifactId>
-            </plugin>
        </plugins>
    </build>

Other information:

[jonenst]

What changes might users need to make in their application due to this PR? (migration steps)

remove build-helper-maven-plugin execution when using it for groovydoc attaching with type javadoc