grafana has no default source for prometheus and alertmanager

Today, our Grafana setup has no default source that would let it point to Prometheus and Alertmanager.

The history is the following:

initially those default source were enabled via the Helm chart
!5815 (merged) moved the configuration for these sources to a Secret
- ... but without removing their configuration via the Helm chart, which created problems
so !6021 (merged) was merged to disable the Helm-based configuration
a few days afterwards !5815 (merged) was reverted (because of other problems)
... but nothing was done to reinstate the Helm-based configuration