[release] Update kubectl plugin doc to beta

[kevin85421]

Search before asking

• I had searched in the issues and found no similar feature requirement.

Description

https://docs.ray.io/en/latest/cluster/kubernetes/user-guides/kubectl-plugin.html

Use case

No response

Related issues

No response

Are you willing to submit a PR?

• Yes I am willing to submit a PR!

[MortalHappiness]

I found it is already beta on the master branch https://docs.ray.io/en/master/cluster/kubernetes/user-guides/kubectl-plugin.html

[kevin85421]

ray-project/ray#50598 (review)