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).

test/TEST-19-DELEGATE/testsuite.sh

@@ -33,7 +33,7 @@ if grep -q cgroup2 /proc/filesystems ; then
         # And now check again, "io" should have vanished
         grep -qv io /sys/fs/cgroup/system.slice/cgroup.controllers
 else
-        echo "Skipping TEST-19-DELEGATE, as the kernel doesn't actually support cgroupsv2" >&2
+        echo "Skipping TEST-19-DELEGATE, as the kernel doesn't actually support cgroup v2" >&2
 fi
 
 echo OK > /testok