[promtail] chart version 3.9.0 is missing from repo index

Hi!

It looks like something broke during the release of the new Promtail 3.9.0 chart. [This commit](https://github.com/grafana/helm-charts/commit/6f3944d109da0859bc9d9fa8d9433ba8a8ef64e7) that was created via Github Actions adds it, but then the [next one](https://github.com/grafana/helm-charts/commit/1cdf1c4c791100f587a6ea765634139cffd3ef3f) removes it.