Update Grails Documentation - Publishing Plugin

The current documentation describes publishing plugin to Bintray which is shutdown already, so we need to update the information to how to publish to Maven Central.

Locate the Grails documentation for the Publishing Plugin and review the section describing Bintray publication. Research the current Maven Central publishing process, then replace the obsolete Bintray guidance with accurate instructions; done means the documentation explains how to publish to Maven Central without referring to the shut-down service.

Written by the indexing model from the issue text.