[DependencyInjection] `ContainerInterface` Deprecation

[etshy]

Follow up of the issue #13469 to (a bit late) give my opinion on it.

This was about the presence in "subscribers and locator" 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