auto-update (PR) github actions

[tbugfinder]

Is your feature request related to a problem? Please describe.
Github actions get updated from time to time by respective maintainers.
It's tedious to follow up on updated github action versions. Therefore it would be beneficial to integrate an action which creates PRs to the actions maintaining action versions.

Describe the solution you'd like
Use tooling like dependabout or https://github.com/marketplace/actions/github-actions-version-updater.

Though it depends on a dedicated access token.

Describe alternatives you've considered
Manual updates.

Additional context
E.g. the checkout@v2 already shows deprecated messages.