Undeprecate IAppContainer and IServerContainer

With 4152216b these two interfaces got
deprecated with the reasoning that we only need the base PSR interface.
However, there are cases where in Nextcloud you still want to have a
specific container (the one for the app vs the one for the server) when
you either have a container injected or query one from a container.

With a single interface that would not be possible. So it's probably
better if we leave the two interfaces, but only have them extend the PSR
interface. IContainer – with the custom methods – shall still be phased
out, but the two other sub interfaces can stay for tagging purposes.
Tagging means that no methods shall be added.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>