fix(api): drop the enum from a status field

[slauger]

Summary

ScaledDownWorkloadRef.Kind carried +kubebuilder:validation:Enum=Deployment;StatefulSet;CronJob, and that field is part of status.

The API server validates status subresource writes. A kind outside the list would therefore have rejected the entire status update, not just that one entry: conditions, phase, the counter and the workload list would all have stopped being written, and the symptom would have looked like an unrelated status bug rather than a validation failure.

Nothing writes an unexpected kind today, so this was latent. It would have become live the moment any work touched the set of supported workload kinds, which is exactly when nobody would be looking for a validation problem.

The same constraint on spec.targets is correct and stays. Validate what users write, describe what the controller reports.

Closes #66

Test plan

Both directions checked against a real API server via envtest:

• The status now accepts an unlisted kind, and the rest of the update lands with it — the test asserts the phase and both list entries persist, since a partial write would be its own bug.
• The spec still rejects an unknown target, so the useful validation is intact.
• Verified the guard works by temporarily reinstating the marker: the new test then fails with status.activeScaledDown[1].kind: Unsupported value, and the whole write is rejected. That is precisely the behaviour being prevented.
• make ci passes.