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/Apps/VerifyCredentials.php

@@ -39,6 +39,6 @@ class VerifyCredentials extends BaseApi
 			DI::mstdnError()->Unauthorized();
 		}
 
-		System::jsonExit(DI::mstdnApplication()->createFromApplicationId($application['id']));
+		$this->jsonExit(DI::mstdnApplication()->createFromApplicationId($application['id']));
 	}
 }