Support psr/container 2

[othercorey]

We're hoping the 2.4/2.5 release can support psr/container 2 since it should be BC with 1.1.

https://github.com/php-fig/container/releases/tag/2.0.0

The only change is an added return type. Other projects are switching to psr/container and depending tend to his this package.

[symfony-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 contracts repository. It is used by Composer to allow developers to depend on specific Symfony contracts.

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

https://github.com/symfony/contracts

Thank you for your contribution!

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; only features should be submitted against the main branch.