Add return type declaration of load method

[franciscosantamaria]

Hi, trying to remove all my deprecation messages in order to upgrade my application to Symfony , it shows me this message:

Method "Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load()" might add "void" as a native return type declaration in the future. Do the same in implementation "Minishlink\Bundle\WebPushBundle\DependencyInjection\MinishlinkWebPushExtension" now to avoid errors or add an explicit @return annotation to suppress this message.

it's safer to add the void return type to the load method

[Minishlink]

Thanks!