Evaluate an auto update branch solution

[mfranzke]

Candidates:

• https://github.com/tibdex/auto-update
• https://github.com/marketplace/actions/autoupdate-branch
• https://github.com/marketplace/actions/auto-update
• https://github.com/marketplace/actions/rebase-pulls

Background:
We don't want to manually update (even not per "Auto update" button) the PRs, especially the ones that are set to auto-merge.

result should get integrated via https://github.com/db-ui/mono/issues/184