feat: Document removal of OC_API legacy class

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

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_31.rst

@@ -81,6 +81,7 @@ Removed APIs
     subscribe('notifications:action:execute', (event) => {
 		console.info('Notification action has been executed:', event.notification, event.action)
     })
+- Legacy class ``OC_API`` was moved to a private namepsace. It should not be needed by applications.
 
 Back-end changes
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