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/Api/Mastodon/Reports.php

@@ -82,6 +82,6 @@ class Reports extends BaseApi
 
 		$this->reportRepo->save($report);
 
-		System::jsonExit([]);
+		$this->jsonExit([]);
 	}
 }