Create a grails-plugin-bom

Add a grails-plugin-bom that's released on major Grails releases with plugins that are known to work with it. During release upgrades, people can update to the bom and if the version is absent they know the plugin isn't updated.

Start by examining the repository's existing plugin metadata and release entry points to understand how Grails releases and compatible plugins are represented. Define the BOM's contents, versioning, and release process for major Grails releases; done means a published BOM can identify plugins known to work with each release.

Written by the indexing model from the issue text.