Enable OCI chart push

[jkroepke]

Fixes #2395

This PR start to publish OCI helm charts to ghcr.io/grafana/charts/, e.g. : ghcr.io/grafana/charts/grafana

Bitnami recently switches to OCI as default source for helm charts

• https://blog.bitnami.com/2023/04/httpsblog.bitnami.com202304bitnami-helm-charts-now-oci.html

and prometheus-community enable OCI based helm charts, too:

• Re-enable publish charts on ghcr.io as OCI artifact prometheus-community/helm-charts#3433

Fetch charts over OCI has multiple benefits:

• It's not longer necessary to download the 1.6M chart index.yaml each time.
• Mirroring to air-gapped environments would much easier, since existing service like Azure Container Registry, Elastic Container Registry and GoHarbor can be used to store/cache the helm charts.

I'm aware that this PR might be a bit aggressive, but I also would like to trigger the discussion here. As I know, this needs to be done at the loki chart, too.

[jkroepke]

@grafana/helm-charts-admins what did you thing?

[zanhsieh]

@jkroepke
I think they need to set up grafana owned ghcr account and put it in Settings > Secerts and variables > Actions > Repository secrets.

[jkroepke]

@zanhsieh No. The GITHUB_TOKEN from the gh actions is sufficient. No additional account is needed.

[zanhsieh]

@jkroepke
Got this on release workflow message:

https://github.com/grafana/helm-charts/actions/runs/5348497649/jobs/9698442262

Run shopt -s nullglob
Error: failed to authorize: failed to fetch anonymous token: unexpected status from GET request to https://ghcr.io/token?scope=repository%3Agrafana%2Fcharts%2Ftempo-distributed%3Apull&scope=repository%3Agrafana%2Fcharts%2Ftempo-distributed%3Apull%2Cpush&service=ghcr.io: 403 Forbidden
Error: Process completed with exit code 1.

If we can't fix it, please consider another PR to rollback.