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/Notifications/Clear.php

@@ -37,6 +37,6 @@ class Clear extends BaseApi
 
 		DI::notification()->setAllDismissedForUser($uid);
 
-		System::jsonExit([]);
+		$this->jsonExit([]);
 	}
 }