Add the finalizer finalizers.multiarch.openshift.io/no-pod-placement-config when the first PodPlacementConfig is created and remove it when the last pod placement config is removed.

Acceptance criteria:

• The enhancement proposal is updated to document this finalizer's role: avoiding the removal of the ClusterPodPlacementConfig until all the PodPlacementConfig objects are deleted.
• [nice-to-have] [if-needed-to-improve-uninstallation-ux] the validating webhook of the cluster pod placement config prints a warning instead of denying the deletion of the cluster pod placement config until all the PodPlacementConfig objects are deleted.
• The finalizer is added/removed according to the description above
• Integration, unit and e2e tests
• Manually test the uninstallation via UI and CLI and ensure the uninstallation transaction succeeds in the possible cases.