Revert "cgroup: do not create a sub-cgroup by default"

[flouthoc]

Reverts #1768

Summary by Sourcery

Restore the default systemd sub-cgroup behavior by returning "container" when no subgroup is specified and update documentation to match the restored defaults.

Bug Fixes:

• Reinstate default sub-cgroup creation by returning "container" instead of NULL in find_systemd_subgroup.

Documentation:

• Update man page and markdown to document default SUBGROUP as "container" on cgroup v2 and empty on cgroup v1.

[sourcery-ai]

Reviewer's Guide

This PR reverts the previous change that disabled default sub-cgroup creation by restoring the fallback to “container” in the code and updating the man pages to document the default behavior and usage across cgroup v1 and v2.

File-Level Changes

Change	Details	Files
Restore default systemd sub-cgroup creation logic	Changed find_systemd_subgroup() fallback from NULL to “container” Reintroduced default SUBGROUP when no annotation is provided	src/libcrun/cgroup-systemd.c
Update documentation for run.oci.systemd.subgroup option	Revised description to explain overriding and default behavior Added default values for cgroup v2 (container) and v1 (empty string) Updated usage examples to match restored defaults Removed outdated paragraphs about no sub-cgroup by default	crun.1 crun.1.md

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[sourcery-ai]

Hey there - I've reviewed your changes and they look great!

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[flouthoc]

Oops I created a revert PR using github UI it doesn't add sign-offs. @giuseppe Might wanna do this with manual PR ?

[packit-as-a-service]

TMT tests failed. @containers/packit-build please check.