Update existing addColaborator to meet Github specs

Update the existing method addCollaborator to meet the current docs.
https://docs.github.com/en/rest/collaborators/collaborators#add-a-repository-collaborator

Instead of NO_CONTENT, the expected status code is CREATED.

lib/src/common/repos_service.dart

@@ -329,16 +329,17 @@ class RepositoriesService extends Service {
     return false;
   }
 
+  /// https://docs.github.com/en/rest/collaborators/collaborators#add-a-repository-collaborator
   Future<bool> addCollaborator(RepositorySlug slug, String user) async {
     ArgumentError.checkNotNull(slug);
     ArgumentError.checkNotNull(user);
     return github
         .request(
           'PUT',
           '/repos/${slug.fullName}/collaborators/$user',
-          statusCode: StatusCodes.NO_CONTENT,
+          statusCode: StatusCodes.CREATED,
         )
-        .then((response) => response.statusCode == StatusCodes.NO_CONTENT);
+        .then((response) => response.statusCode == StatusCodes.CREATED);
   }
 
   Future<bool> removeCollaborator(RepositorySlug slug, String user) async {