[GTIN] Hide the Global Trade Item Number field for new users and make read-only for existing users

[martynmjones]

As we will be prioritising the global unique ID field in core (See #2614) when syncing product data, the GTIN field in Google for WooCommerce will need to be hidden for new users of the extension and made read-only for existing users. The tooltip will also need to be updated to direct existing users to the core Inventory tab to popular the field.

Acceptance criteria

1. For new merchants, the GTIN field is automatically hidden in both the classic and new product editor
2. For existing merchants, the GTIN field is made read-only and the tooltip directs them to the Inventory tab

Additional

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