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/Statistics.php

@@ -83,6 +83,6 @@ class Statistics extends BaseModule
 		], $services);
 
 		$this->logger->debug("statistics.", ['statistics' => $statistics]);
-		System::jsonExit($statistics);
+		$this->jsonExit($statistics);
 	}
 }