Fix PHP 8.4 implicitly nullable deprecation

[acrobat]

Fix the Implicitly marking parameter XXX as nullable is deprecated, the explicit nullable type must be used instead deprecation in PHP 8.4

This PR also enables to CI build against php 8.3 and 8.4

[wouterj]

Thanks @acrobat!

[dannyvw]

@wouterj Is it possible to create a new release with this fixes?

[wouterj]

Given this is only a deprecation notice, so nothing breaking, I prefer to wait a bit for a more significant change. Otherwise, we're burning network resources from everyone updating for no benefit :)

Let's see if work is needed for Symfony 7.3 compatibility or maybe finish #123

[a24710]

+1 to a new release for this please. We are upgrading to php 8.4 and we would like to keep the warnings as close to zero as possible.