feat: Document removal of OCP\Files\Storage in 31

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

[skip ci]

Author: come-nc

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_31.rst

@@ -83,6 +83,7 @@ Removed APIs
     })
 
 - Legacy class ``OC_API`` was moved to a private namespace. It should not be needed by applications.
+- Deprecated interface ``OCP\Files\Storage`` was removed. Use ``OCP\Files\Storage\IStorage`` instead.
 
 Back-end changes
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