To avoid confusion, rename gcp_project_id to gcp_cluster_project_id

[Glenn-Terjesen]

https://github.com/entur/gha-meta/blob/cf4f1c2a8a8c22ac05451ffddc5e77dc6afc4380/.github/actions/k8s-auth/action.yml#L10C3-L10C17

and improve description - this is usually only for team plattform ( ie: do not use )