cgroup: s/cgroups? ?v?([0-9])/cgroup v\1/gI

cgroup: s/cgroups? ?v?([0-9])/cgroup v\1/gI

Nitpicky, but we've used a lot of random spacings and names in the past,
but we're trying to be completely consistent on "cgroup vN" now.

Generated by `fd -0 | xargs -0 -n1 sed -ri --follow-symlinks 's/cgroups?  ?v?([0-9])/cgroup v\1/gI'`.

I manually ignored places where it's not appropriate to replace (eg.
"cgroup2" fstype and in src/shared/linux).