Support new scheduler-plugins API group

[tenzen-y]

Since kubernetes-sigs/scheduler-plugins#526, the scheduler-plugins switch the API group to x-k8s.io.
So we need to support both API groups to keep backward compatibility since we are using an old API group, sigs.k8s.io.

/kind feature

[tenzen-y]

With more investigation, it is hard to keep backward compatibility since the scheduler-plugins fully replaced the sigs.k8s.io with the x-k8s.io.

Maybe, we need to stop supporting v0.24.x or lower scheduler-plugins.

@johnugeorge, Can you add a notice that we will stop supporting older scheduler-plugins v0.24.x or lower in the next training-operator version (v1.7.0) to our release note for v1.6.0?

[tenzen-y]

ref: kubeflow/mpi-operator#500

[johnugeorge]

Makes sense. It is hard and I think, it is ok. Added the note to the release

[tenzen-y]

Makes sense. It is hard and I think, it is ok. Added the note to the release

Thanks!

[tenzen-y]

/assign

[tenzen-y]

I will work on this after #1714 is completed.