Update k8s resolver to work with cluster-client v2

What does this MR do and why?

As the new major version of the @gitlab/cluster-client library uses fetch instead of axios, we'll need to update the related logic for the changes in request and response data.

Screenshots or screen recordings

No visual changes, screenshots are for the context.

How to set up and validate locally

The updated version is not yet available. To verify this MR, check out the related MR - feat!: Update generator from axios to fetch (gitlab-org/cluster-integration/javascript-client!9 - merged). Build the javascript client and link it to the main GitLab project via yarn link.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422945 (closed), #427865 (closed)