[admin] Setup Github App for automated renovate dependencies

[jkroepke]

In #4292 we decide to evaluate renovate for dependency updates.

However, renovate is not able to bump the chart version which is manatory for an release. (ref: renovatebot/renovate#28045)

After almost one year, I decide to setup an companion workflow which bump the version for renovate. (source: https://github.com/prometheus-community/helm-charts/blob/main/.github/workflows/renovate-bump-chart-version.yaml)

The workflow work fine, however due a github action limitation, commits from the action user doesn't trigger workflow runs again.

In action: #5201
Docu: https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/triggering-a-workflow#triggering-a-workflow-from-a-workflow

As documented, an GitHub App which can be used inside GitHub actions to commit from GitHub action.

Instructions

A GitHub app can be created on org level: https://github.com/organizations/creativesandbox/settings/apps/new

Callback URL, Webhook and Setup URL can be empty. For permissions, Metadata read and Contents write.

After the app is created, the app can be installed on the org. On installation, the apps permissions can be limited to one repository. In this case prometheus-community/helm-charts.

In order to use the app from actions, the App ID and the private key (raw value, including new lines) needs to be stored as secrets within the repository.