Permissive event dispatcher contracts for 4.4

[alexpott]

Q	A
Branch?	4.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #...
License	MIT
Doc PR	symfony/symfony-docs#...

Allows the event-dispatcher component to be used on PHP 8.

[alexpott]

This is not going to work...

+/home/travis/build/symfony/symfony/phpunit --exclude-group tty,benchmark,intl-data,legacy
#!/usr/bin/env php
PHP Fatal error:  Declaration of Symfony\Component\EventDispatcher\EventDispatcher::dispatch($event) must be compatible with Symfony\Contracts\EventDispatcher\EventDispatcherInterface::dispatch(object $event, ?string $eventName = NULL): object in /home/travis/build/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php on line 33
Fatal error: Declaration of Symfony\Component\EventDispatcher\EventDispatcher::dispatch($event) must be compatible with Symfony\Contracts\EventDispatcher\EventDispatcherInterface::dispatch(object $event, ?string $eventName = NULL): object in /home/travis/build/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php on line 33
KO  7.3 src/Symfony/Component/EventDispatcher 

https://travis-ci.org/github/symfony/symfony/builds/705336146

So it'd be great if #37494 can be backported to v1.