update kubernetes-client version to 5.12.1

[kyoppii13]

Updated version of k8s client to 5.12.1.

The reason is as follows.

• I want to use the backoff parameter (kubernetes.request.retry.backoffLimit).
• Since the supported version of k8s cluster was old.

https://github.com/fabric8io/kubernetes-client

kubernetes-client migration document fromt 4.x to 5.x

[yoyama]

LGTM. But it might be better to add some tests.

[kyoppii13]

@yoyama Thanks for your review. I added a test. please review again.

[kyoppii13]

@yoyama I resolve conflicts. Please review.

[yoyama]

@kyoppii13 I left comments on minor things. Please check it.

[kyoppii13]

@yoyama Thanks for your review. I fixed it. Please review.