Use external static ip instead of ephemeral for GKE clusters

Description

When adding a load balancer (e.g., nginx-ingress) to your Kubernetes cluster on GKE, you get an external ip that is ephemeral. This means it may change in the future, and this is something not so good for production use.

GCP gives you the ability to "promote" your ephemeral ip into a static one (https://cloud.google.com/compute/docs/ip-addresses/reserve-static-external-ip-address), and this can be made easy for GitLab users from the CI/CD > Kubernetes page.

This is important to guarantee the ip never changes, and people can rely on this information to set up production environments where application can be deployed and run in a long-term way.

Proposal

When installing the nginx-ingress, configure it in order to use a static ip address instead of an ephemeral one.

If possible, this should be done during the installation process, otherwise we could install as usual and then automatically promote the ephemeral ip into a static one as soon as the installation completes.

We should add a link in the section that deletes a cluster integration bringing to docs on how to clean up everything on GKE (how to delete the cluster on GKE, how to delete the external static ip).