Add (semi-)automatic updating of HelmReleases

[dsluijk]

Right now all HelmReleases are pinned to a minor version. We should add a way to semi-automatically update these releases to the latest version. I think it would be best if we automatically create PR's with these updates, as some might have breaking changes. This allows us to check out the changelog before merging.

Some documentation on this: https://fluxcd.io/flux/use-cases/gh-actions-helm-promotion/.