upgrade from 1.1.1 broken because of removal of source_templates.x.existing_source

https://gitlab.com/sylva-projects/sylva-core/-/jobs/7605579733#L547:

Following resources were not ready when exit condition matched:
    * HelmRelease/sylva-units: UpgradeFailed - Helm upgrade failed for release sylva-system/sylva-units with chart sylva-units@0.0.0-git+7cd1d5afe411.2: values don't meet the specifications of the schema(s) in the following chart(s):
sylva-units:
- source_templates.sylva-core: Additional property existing_source is not allowed

This is due to !2694 (merged) which introduced a change in sylva-units values schema.