[Symfony 5] Symfony/Component/EventDispatcher/Event is deprecated in Symfony 4.3 use Symfony/Contracts/EventDispatcher/Event instead

Problem/Motivation

Symfony/Component/EventDispatcher/Event is deprecated in Symfony 4.3 use Symfony/Contracts/EventDispatcher/Event instead

Proposed resolution

Create a new Drupal\Component\EventDispatcher\Event class so that contrib/custom/core core can extend it. When Drupal 10 upgrades to Symfony 5, we can change the inheritance of that class to the one from Symfony Contracts, but contrib and custom code won't need to update since they'll only be referencing the Drupal version directly.

Remaining tasks

Because the deprecation is from Symfony, and Symfony itself is still using the deprecated event class, it is either hard or impossible to actually remove the deprecation message suppression.

User interface changes

API changes

Data model changes

Release notes snippet