Move System::jsonExit to BaseModule->jsonExit

- This will ensure headers set in BaseModule->run will be carried in jsonExit scenarios
- Deprecate jsonExit() method in Core\System

src/Module/BaseNotifications.php

@@ -121,7 +121,7 @@ abstract class BaseNotifications extends BaseModule
 			'page'          => $pager->getPage(),
 		];
 
-		System::jsonExit($notifications);
+		$this->jsonExit($notifications);
 	}
 
 	/**