Kubernetes API version (apps/v1beta1) should be updated to apps/v1

[oke-py]

Statefulset in apps/v1beta1 is deprecated in Kubernetes v1.16.
https://kubernetes.io/blog/2019/07/18/api-deprecations-in-1-16/

It is used as below.

$ git grep apps/v1beta1                          
example/StatefulSet/mongo-statefulset.yaml:apiVersion: apps/v1beta1

Currently, GKE is using v1.14 or v1.15, and so these manifests can be applied.
However, a few months later, GKE will use v1.16, and then the manifests cannot be applied.