fix: postgresql subchart

[jon-whit]

Description

In #89 I broke the Helm chart for those users who want to quickly deploy Postgres alongside the OpenFGA service. This fixes that. I verified the commands in the README work.

References

#89

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected

[evankanderson]

Hi, this seems to have renamed the postgres key to postgresql on a minor version update. This seems like a breaking change. I realize that the chart is at 0.1.39, so that could be expected, but I wanted to let you know that we're actually using this to deploy OpenFGA.

[evankanderson]

We can change the key, but our ArgoCD deployment is currently in state Error with the following complaint:

Manifest generation error (cached):

`helm template . --name-template openfga --namespace openfga --kube-version 1.27 --values /tmp/3db5fc69-a673-4c62-b897-d09920b9b4c0 <api versions removed> --include-crds` failed exit status 1: Error: values don't meet the specifications of the schema(s) in the following chart(s): openfga: - (root): Additional property postgres is not allowed

Let us know whether we should switch to the new key or if the rename is going to be rolled back.