[chore] Update celery beat schedule for user expiration logic

[pandafy]

With the planned changes to user expiration logic in the following issues:

• openwisp-users#499
• openwisp-radius#706

We need to update the Celery beat schedule configuration to use the correct celery task exposed by openwisp-users. The variable name cron_deactivate_expired_users will remain the same to avoid any breaking change for users. Please adjust the implementation and documentation accordingly if needed.

Acceptance Criteria:

• The Celery beat schedule uses the updated celery task from openwisp-users for deactivating expired users.
• The variable name cron_deactivate_expired_users remains unchanged.
• No breaking change for Ansible role users.
• Update docs or comments if the celery task name/reference changes.

See linked issues for more details on the changes.