Use Symfony's new contract Event class instead of the deprecated one

[ChristophWurst]

Leftover from the 4.3 bump at #17049.

Fixes the

deprecated the Event class, use Symfony\Contracts\EventDispatcher\Event instead

deprecation for the new event dispatcher system. In theory this could be a breaking change as apps might have used methods of the old base class, but the likelihood is very low IMO.

[rullzer]

Acceptable IMO.

[ChristophWurst]

This now includes the changes from #17084 as dispatch would otherwise do 💥 if it detects the usage of the new event base class.

[nickvergessen]

Which methods are affected? isPropagationStopped and stopPropagation?
If so sharing and talk chat message parsing are affected

[ChristophWurst]

Do they use Symfony directly or our abstraction?

[rullzer]

Both functions are still there.

[rullzer]

Which methods are affected? isPropagationStopped and stopPropagation?
If so sharing and talk chat message parsing are affected

Also note that it is only for things that use the new event stuff. So talk is not affected right now.

[ChristophWurst]

Pushed another commit. 🤞 this makes our tests happy. Apparently we ran into a form of the fragile base class problem. Yay.

[rullzer]

rebase