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/InstanceV2.php

@@ -94,7 +94,7 @@ class InstanceV2 extends BaseApi
 		$contact              = $this->buildContactInfo();
 		$friendica_extensions = $this->buildFriendicaExtensionInfo();
 		$rules                = System::getRules();
-		System::jsonExit(new InstanceEntity(
+		$this->jsonExit(new InstanceEntity(
 			$domain,
 			$title,
 			$version,