Setters with a default value are a bit odd: I want to set a value, but I won't tell you which one!

We do have this kind of setters, like this example from TokenStorageInterface:

public function setToken(TokenInterface $token = null);

This means that those to calls are equivalent:

$tokenStorage->setToken(null);
$tokenStorage->setToken();

The reason for this is actually a php 5 quirk: On php 5, we did not have nullable parameter type declarations – those we added in php 7.1. The only workaround was to declare null as default value because then php would accept passing null despite the type declaration demanding an instance of a specific class/interface.

Because the days of php 5 are over, I'd like to change this. Our method signature would then look like this.

public function setToken(?TokenInterface $token);

We can do this for interfaces and abstract methods because an implementation may add a default value. Thus, removing the default value from the interface alone is not a BC break.

For the implementations of that interface, this is a different story because removing the default breaks calling code that omits the parameter entirely. This is why for the implementations I trigger a deprecation if the method is called without arguments. This enables us to remove the default in Symfony 6.

This PR performs the suggested changes for TokenStorageInterface and ContainerAwareInterface, but we have a few more setters like this. But before I continue, I'd like to collect some feedback if this is something you would want to change.