Move grails-fields-plugin to Grails Plugin Collective

[sbglasius]

I suggest moving the grails-fields-plugin to The Grails Plugin Collective (https://github.com/gpc)

Reasons:

• There is already a build process using GitHub actions.
• The GPC is already home to several Grails plugins
• There is a common (new) group (io.github.gpc) for plugins released under the GPC. No need to apply for a new Sonartype deploy key, gpg key etc.

@jeffbrown @puneetbehl please weigh in.

[aadrian]

Scaffolding depends on this, so the main group https://github.com/grails would be a better place.
(The GPC seems to contain only less used plug-ins - none where essential functionality depend on it)

[puneetbehl]

I do not have sufficient permissions with GPC to do the move. See following:

Once you grant me the required permissions. Please drop me a messasge.