GitLab.com for Jira Cloud app connected to self-managed can only link public namespaces
### Summary
Some users report that only public namespaces can be linked to the in the GitLab.com for Jira Cloud app if connected to a self-managed instance.
Related issues:
- https://gitlab.com/gitlab-org/gitlab/-/issues/388152
- https://gitlab.com/gitlab-org/gitlab/-/issues/386827
### Steps to reproduce
1. Install the app for a self-managed instance.
2. Log out of the self-managed instance.
3. See only public namespaces in the app.
### Relevant logs and/or screenshots
It looks like the auth header is not included in the request fetching namespaces. (https://gitlab.com/gitlab-org/gitlab/-/issues/386827#note_1232318911)
### Workaround
*Note: This workaround might not be feasible for all users*
1. Make the namespace public.
2. Link the namespace.
3. Make the namespace private again.
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