don't use bootconfig for systemd's unified cgroup hierarchy about bottlerocket HOT 1 CLOSED

While testing #3935 I discovered that it's actually possible to use bootconfig to handle the aws-k8s-1.25 case I was worried about.

Kernel command line parameters are divided into two sets - one for the kernel, one for init - and normally these trump anything specified in bootconfig. The kernel will pass unrecognized parameters along to init, under the assumption that they might be intended for that process. However, if bootconfig has a value for an init parameter of the same name, then that will take precedence over the value passed to init, because all init parameters come after all kernel parameters.

So if systemd.unified_cgroup_hierarchy=0 is set as a kernel parameter in the variant definition - even though it is an init parameter - it can be overridden by a bootconfig parameter at runtime.

from bottlerocket.