kanister-kubectl docker image maintenance

[PrasadG193]

As of now, we can see multiple kanister-kubectl images in the ghcr registry

• ghcr.io/kanisterio/kanister-kubectl - we don't build and publish this as a part of the release pipeline
• ghcr.io/kanisterio/kanister-kubectl-1.18

Going forward, in order to support recent versions of kubectl, we need to decide on the consistent image format which would be easier to maintain and extensible.

[pavannd1]

I think we should use ghcr.io/kanisterio/kanister-kubectl and keep it updated to the latest version of kubectl.
For special requirements, we can build separate images like ghcr.io/kanisterio/kanister-kubectl-1.18