[DependencyInjection] `ContainerInterface` Deprecation

Follow up of the issue #13469 to (a bit late) give my opinion on it.

This was about the presence in "[subscribers and locator](https://.com/symfony/symfony-docs/blob/master/service_container/service_subscribers_locators.rst#defining-a-service-subscriber)" doc where we can still see `ContainerInterface` being used.

> I know we don't have the full container in a locator, but why not recommending using `ServiceProviderInterface`, the direct interface of `ServiceLocator`, which extends `ContainerInterface` ?
> 
> It would be less confusing about the deprecation of `ContainerInterface` usage but still seeing it on some part of the doc, even if unrelated.

Should we change the locator doc to show `ServiceProviderInterface`

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