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/Instance.php

@@ -59,6 +59,6 @@ class Instance extends BaseApi
 	 */
 	protected function rawContent(array $request = [])
 	{
-		System::jsonExit(new InstanceEntity($this->config, $this->baseUrl, $this->database, System::getRules()));
+		$this->jsonExit(new InstanceEntity($this->config, $this->baseUrl, $this->database, System::getRules()));
 	}
 }