Downgrade Attribute\AsEventListener to PHP 7.4

[phpfui]

Attribute\AsEventListener is an 8.0 file. It is barfing in my project dependencies for 7.4. Downgraded it to 7.4 compatible code.

[symfony-bot[bot]]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony repository. It is used by Composer to allow developers to depend on specific Symfony components.

If you want to contribute, you should instead open a pull request on the main repository:

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PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply.