automatic updates when helm chart is published

[minrk]

we should have automatic PRs to update dependency charts, like https://github.com/jupyterhub/mybinder.org-deploy/blob/main/.github/workflows/watch-dependencies.yaml

[minrk]

apparently dependabot helm opens broken PRs for versions that don't exist:

• helm renders prerelease version incorrectly dependabot/dependabot-core#15171
• Helm: Incorrect filtering of prerelease versions for OCI registry dependencies dependabot/dependabot-core#14025