Fix permission check when setting issue/merge request subscription in GraphQL API.

This change was first introduced in merge request
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61980.

Changelog: fixed

changelogs/unreleased/fix_set_subscription_permission.yml

----
-title: Fix permission check when setting issue/merge request subscription in GraphQL
-  API.
-merge_request: 61980
-author:
-type: fixed