Fix Zend/Barcode composer.json to require zendframework/zend-validator.

As per #6293, make `zendframework/zend-barcode` require `zendframework/zend-validator`, since barcodes are validated at draw time. We also mention why `zendframework/zend-servicemanager` is suggested.

I have not been able to test this yet (outside of validating the composer file), because I would first need to split Zend/Barcode into a subtree to feed it to Composer, and set-up a VM to get a recent enough version of PHP. Feel free to take it as-is or to wait until I test it.