[6.x] Don't reset revisions when saving a collection with the field hidden

[lazerg]

Saving a collection's config writes revisions: false into the collection YAML on any instance where revisions are switched off, even though the toggle was never on screen. If a site keeps STATAMIC_REVISIONS_ENABLED true in production and false locally, editing anything else in the collection config from local quietly turns revisions off for everyone.

The revisions field only gets added to the config blueprint when statamic.revisions.enabled is on and the site is Pro. Without it, the submitted values carry no revisions key at all, and the update fell back to $values['revisions'] ?? false.

The setter now only runs when the field was part of the form, so a hidden toggle leaves whatever is already in the YAML alone.

There's a related one I've left alone: with revisions on, defaultPublishState() always reports false, so the config form posts that value back and the collection picks up default_status: draft on a save that changed nothing. That needs a decision about what the getter should return rather than a guard, so it felt like a separate PR. Happy to pick it up if you want it.

Fixes #15430