Fix admin_mode enforcement in AdjournedProjectDeletionWorker

This change addresses an issue where project deletion fails when
scheduled by an admin with admin mode enabled. The problem occurs
because the `admin_mode` session value is not persisted to Sidekiq.

To resolve this, we introduce an `admin_mode` flag for
`ProjectDestroyWorker`, creating a temporary admin session for
project deletion. This approach ensures that other policy checks,
such as blocking or user permissions, remain enforced while
bypassing the admin_mode check. The admin_mode check should only be
performed when the deletion is scheduled. We have previously
implemented a similar solution for groups in a separate merge
request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163071.

Changelog: fixed
EE: true