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

We also explain why zendframework/zend-servicemanager is suggested.

zendframework/zend-validator is needed because all barcodes are validated using
Zend\Validator\Barcode prior to drawing.

zendframework/zend-servicemanager is suggested because it’s needed when using
the factory methods to create or render a barcode.
Merge branch 'hotfix/#6310-fix-#6293-zend-barcode-dependencies'

Close #6310
Close #6293