[ticket/11854] Move captchas to phpbb/ and use DI for plugins

https://tracker.phpbb.com/browse/PHPBB3-11854

TODO:
- [x] Migration to update the current setting

PHPBB3-11854
you prefer to have to write the service name twice in the service definition?
ie:
```
    captcha.plugins.service_collection:
        class: phpbb\di\service_collection
        arguments:
            - @service_container
        calls:
            - [set_name, [captcha.plugins.service_collection]]
        tags:
            - { name: service_collection, tag: captcha.plugins }
```