GCP auth plugin will be unavailable in K8s 1.25

[mbelchev]

Is your feature request related to a problem?

The plugin outputs the following warning message due to the introduced changes from the GCP side in regard to the GKE authentication (ref: https://cloud.google.com/blog/products/containers-kubernetes/kubectl-auth-changes-in-gke):
WARNING: the gcp auth plugin is deprecated in v1.22+, unavailable in v1.25+; use gcloud instead.

Therefore it'll become a problem for some users using the newer Kubernetes versions.

Describe the solution you'd like
The official solution, provided by GCP is to set the USE_GKE_GCLOUD_AUTH_PLUGIN environment variable to True (check the ref doc above).

This can be set in the plugin's image (https://github.com/nytimes/drone-gke/blob/main/Dockerfile#L7) by default. But it has to be double-checked how it affects the previous Kubernetes versions. Maybe it's worth adding some logic to conditionally enable it based on some version detection at run time.