Follow-up from "Automate bump version k8s Workloads"

With the way it's currently done, a slight change in indentation or the section ordering may break this. Can we use a YAML lib to:

1. Unmarshal YAML file to struct
2. Modify the wanted <env>.values.registry_version field in place
3. Marshal struct back to YAML
4. Write file

We're already using gopkg.in/yaml.v2 in the codebase, so we can rely on that.

from: !1120 (comment 1131280446)