Support for Symonfy 6

[Chris53897]

Allow Symfony 6.
This PR #537 already removes some deprecations.

TODO:

• Change composer.json
• Remove all deprecations (DI)
• Switches for Symfony 4 and 5+6
• Dependencies?
• Adjust Testing
• Tests

I already took a quick look and could get an installable set (but not stable releases). https://github.com/Chris53897/PayumBundle/tree/feature/symfony6

It would help the maintainers to drop Support of Symfony 4.
https://symfony.com/releases/4.4
https://symfony.com/releases
WDYT?
As it is still an active LTS i can understand to not drop support yet.

I can take this PR, but i will wait after some PRs of mine are hopefully merged.
At least #536 for betting Testing.

[pierredup]

Since 4.4 is still in active support for the rest of the year (and then 1 year of security fixes after that), I don't think we should drop support for 4.4 now. If possible, we should try and support from 4.4 to 6.0. If that's not possible, then I think we can start working on version 3 of this bundle, which drops Symfony 4 and PHP 7 support, but still keep version 2 in active support for the next year so (effectively meaning that we have both active and supported version 2 and 3 of this bundle)

[Chris53897]

I guess it will be possible to support both. But a lot of extra work.
And there are BC breakes, example, so a major version makes more sense to me.
I would love to do a PR for version 3 with Php >= 8 and Symfony ^5.4 || ^6.0 Support.

I could start with the PR after my 2 remaining PRs are merged and i have your permission to do this like suggessted.