Add product listener to post update available on date to following master variant

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | #1651
| License       | MIT
| Doc PR        | -
I guess that best would be:
```php
use Sylius\Component\Product\Model\VariantInterface;

//...

$variant = $product->getMasterVariant();
if (!$variant instanceof VariantInterface) {
    return;
}

$product->setAvailableOn($variant->getAvailableOn());