Merge pull request #12998 from nextcloud/backport/12997/stable31

[stable31] feat: Document removal of OC_API legacy class

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_31.rst

@@ -82,6 +82,8 @@ Removed APIs
 		console.info('Notification action has been executed:', event.notification, event.action)
     })
 
+- Legacy class ``OC_API`` was moved to a private namespace. It should not be needed by applications.
+
 Back-end changes
 ----------------