This extension is now using the new way of providing monitoring configuration (ref [GEP-19](https://github.com/gardener/gardener/blob/master/docs/proposals/19-migrating-observability-stack-to-operators.md)) in case a shoot cluster's Prometheus has been migrated to management via `prometheus-operator`.
How to categorize this PR?
/area monitoring /kind enhancement
What this PR does / why we need it: https://github.com/gardener/gardener/pull/9695 changes the contract of how extension can provide monitoring configuration, see https://github.com/gardener/gardener/pull/9695/files#diff-93972179f36fa195b948f0f4979acab75697f99f3c51ab889994d9d8ba9f170b. This PR adapts this extension accordingly. It also cleans up no longer needed configuration (if found).
Which issue(s) this PR fixes: Part of https://github.com/gardener/gardener/issues/9065
Special notes for your reviewer: /cc @ScheererJ
Release note: