utils/systemd: stop using capture_output so Python 3.6 targets keep working

[SAY-5]

What does this PR do?

Replaces the two subprocess.run(..., capture_output=True) calls in salt/utils/systemd.py with the older-compatible stdout=subprocess.PIPE, stderr=subprocess.PIPE form.

capture_output was added in Python 3.7. salt-ssh still advertises 3.6 as a possible target python on older distros (see #68778), so the two call sites in _status() and _pid_to_service_systemctl would raise TypeError the moment they executed on a 3.6 remote — which is exactly what the reporter hits.

Behaviour on 3.7+ is unchanged.

What issues does this PR fix or reference?

Fixes #68778

Previous Behavior

On salt-ssh runs where the remote happens to still be on Python 3.6, any salt.utils.systemd import path that touched _status or _pid_to_service_systemctl raised:

TypeError: __init__() got an unexpected keyword argument 'capture_output'

New Behavior

Both helpers now build the pipes explicitly:

subprocess.run(
    ["systemctl", "status"],
    check=False,
    stdout=subprocess.PIPE,
    stderr=subprocess.PIPE,
)

Merge requirements satisfied?

• Changelog: changelog/68778.fixed.md
• Tests — the two call sites shell out to systemctl, which isn't straightforward to unit-test without an extra fixture. Happy to add one if the maintainers prefer before merge.

Commits signed with GPG?

DCO-signed off (Signed-off-by: SAY-5 <SAY-5@users.noreply.github.com>).

[frebib]

Python 3.6 is a 9 year old release than went EOL in 2021: https://endoflife.date/python

Update your python