Helm: Publish Helm charts to grafana/helm-charts. (#6955)

**What this PR does / why we need it**:
This change pulls in the GitHub workflow for publishing Loki Helm charts to `grafana/helm-charts`.

**Which issue(s) this PR fixes**:
Fixes #6877

.github/workflows/helm-release.yaml

@@ -0,0 +1,20 @@
+name: helm-release
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - "v[0-9]+.[0-9]+.[0-9]+"
+
+jobs:
+  call-update-helm-repo:
+    uses: grafana/helm-charts/.github/workflows/update-helm-repo.yaml@main
+    with:
+      charts_dir: production/helm
+      cr_configfile: production/helm/cr.yaml
+      ct_configfile: production/helm/ct.yaml
+    secrets:
+      helm_repo_token: ${{ secrets.GH_BOT_ACCESS_TOKEN }}
+      # See https://github.com/grafana/helm-charts/blob/main/INTERNAL.md about this key
+      gpg_key_base64: ${{ secrets.HELM_SIGN_KEY_BASE64 }}

production/helm/cr.yaml

@@ -0,0 +1,5 @@
+git-repo: helm-charts
+key: Grafana Loki
+owner: grafana
+sign: true
+skip-existing: true