Merge: Release oncall Helm chart 1.3.15 (#2578)

Merge this PR to `main` branch to start another [github actions
job](https://github.com/grafana/oncall/blob/dev/.github/workflows/helm_release.yml)
that will release the updated version of the chart (version: 1.3.15,
appVersion: v1.3.15) into `grafana/helm-charts` helm repository.

This PR was created automatically by this [github
action](https://github.com/grafana/oncall/blob/dev/.github/workflows/helm_release_pr.yml).

---------

Co-authored-by: GitHub Actions <actions@github.com>

helm/oncall/Chart.yaml

@@ -2,8 +2,8 @@ apiVersion: v2
 name: oncall
 description: Developer-friendly incident response with brilliant Slack integration
 type: application
-version: 1.3.14
-appVersion: v1.3.14
+version: 1.3.15
+appVersion: v1.3.15
 dependencies:
   - name: cert-manager
     version: v1.8.0