255.5-1.3: Don't use pidfds for pids from cgroup.threads
    
    This release backports the following PRs:
    - https://github.com/systemd/systemd/pull/33084
    - https://github.com/systemd/systemd/pull/32534
    
    The second PR is only backported to reduce the number of merge conflicts
    when backporting the first PR, but should be safe to backport on its own.