This will allow us to get rid of the `KUBECONFIG_CONTENTS` CI/CD
var in the API GitLab repo. The config wasn't been used anymore
by the API when deployed to a cluster anyway, instead using a
`ServiceAccount`.

Env var `K8S_CONFIG_PATH` is still available to inject a config when
deploying the API standalone (e.g. for local developoment)

Also when building the API image for production, use bookworm as
the base image instead of buster which is pretty old by now