Remove pod security policy

Pod security policy is deprecated since k8s version 1.21 and scheduled to be removed by 1.25, see the notice here - https://kubernetes.io/blog/2021/04/08/kubernetes-1-21-release-announcement/#podsecuritypolicy-deprecation.

Aside from the removal of the resource itself (which is a breaking change), we should also add some info and recommendations on switching to pod security admission - https://kubernetes.io/docs/concepts/security/pod-security-admission/