Update to Symfony 6.3 and PHP 8.1

[czachor]

Hello,
the bundle wasn't working for me with Symfony 6 and PHP 8.2, so I made the fork. I think it could be connected to #31

What is done:

1. PHP min. version set to 8.1.
2. Symfony min. version set to 6.3. Probably will work with 7.0, but not tested.
3. Controller is extending from Symfony\Bundle\FrameworkBundle\Controller\AbstractController instead of Symfony\Bundle\FrameworkBundle\Controller\AbstractController\Controller (not available in Symfony 6).
4. Removed some deprecations.
5. Code formatting (PSR).
6. Commands: added returns (Command::SUCCESS).
7. A few code micro-optimizations.
8. Changed type comparison operators (!= -> !==).
9. Added function's return types.
10. Added property types.
11. SubscriberEventand WebhookEvent: removed Symfony\Component\EventDispatcher\Event class (removed in Symfony 4?)
12. routing.yaml: changed type: annotation to type: attributes.
13. A lot of small changes related to language level / Symfony version upgrade.
14. WelpMailchimpBundle extends Symfony\Component\HttpKernel\Bundle\AbstractBundle instead of Symfony\Component\HttpKernel\Bundle\Bundle.
15. Added autowiring instead of manually getting services.

TODO:

1. I didn't rebuild the tests: the original ones didn't work too well for me and it's a big TODO.
2. Memory usage: syncing ~4000 users consumes a lot of memory. I'll do some tests, but I think using yield could be helpful.
3. It's been a long time since I wrote a bundle, so there are probably a few things to improve. I'm open to suggestions and of course feel free to modify the code.

If you decide to merge, I thing it should be major release (a'la mentioned above 2.0), there are BC changes here.

Anyway it works perfect with my app syncing ~4000 users with Mailchimp.