Revise custom images documentation

[pebrc]

We use the gcs-repository plugin to illustrate the process of creating custom Docker images here https://www.elastic.co/guide/en/cloud-on-k8s/current/k8s-custom-images.html

As of Elasticsearch 8.0 the common repository plugins (Azure, GCS, S3) are already bundled in the regular image so there is no need to create a custom image for that. We should use a different example as this might be misleading to users and I am not sure if the plugin installation of an already installed plugin would actually succeed.