Replace decommissioned simple-db-app demo in gitops fixtures

[marians]

Context

While cleaning up references to the decommissioned giantswarmpublic.azurecr.io registry (which has been consolidated into oci://gsoci.azurecr.io/charts/giantswarm/), the demo app simple-db-app turned up in several gitops fixtures and templates:

• gitops-template: bases/environments/stages/{dev,staging}/hello_app_cluster/imagerepositories.yaml, ATS assertions under tests/ats/assertions/exists/.../imagerepositories.yaml
• observability-gitops: equivalent ATS assertion fixtures

These reference giantswarmpublic.azurecr.io/giantswarm-catalog/simple-db-app.

Problem

The simple-db-app repository no longer exists (GitHub 404) and the chart is not published to gsoci (charts/giantswarm/simple-db not found). So unlike hello-world, it cannot be migrated 1:1 to the new registry path.

In the registry-cleanup PR, hello-world-app references are migrated to oci://gsoci.azurecr.io/charts/giantswarm/hello-world, but the simple-db-app lines are left untouched (still pointing at the dead registry) pending this decision.

Decision needed

Pick a replacement strategy for the simple-db-app references:

1. Drop the simple-db-app ImageRepository / source blocks entirely (keep only hello-world).
2. Replace with another real, currently-published demo chart so the fixtures still exercise two apps.

Also applies to the mirrored fixtures in giantswarm/observability-gitops.