Remove v1alpha1 in v0.31.0

[ingvagabund]

v1alpha1 version of the descheduling policy has been deprecated for many releases now. It's time to remove it from the code base and focus on v1alpha3.

Removal announced on sig-scheduling dev list: https://groups.google.com/g/kubernetes-sig-scheduling/c/OyFjkDntuhg/m/GVjD4gpaBAAJ?utm_medium=email&utm_source=footer
1.30 k8s release scheduled for Wednesday 17th April 2024: https://www.kubernetes.dev/resources/release/#summary

TODO:

• - remove v1alpha1 types from the code
• - remove v1alpha1 -> v1alpha2 conversion code
• - remove any mention of v1alpha1 type from the documentation (probably just mentioning v1alpha1 was removed in 0.31.0)
• - announce v1alpha1 removal on sig-scheduling dev list + descheduler changelogs

[a7i]

fyi- Warning should land in v0.30 and should be noted in the release notes: #1337