net: remove deprecated net.unbounded-cloud.io/site node label write + fallback

During the `unbounded-system` consolidation, node site-membership moved from the deprecated `net.unbounded-cloud.io/site` label to the canonical `unbounded-cloud.io/site` label.

For the deprecation window:
- unbounded-net **dual-writes** both labels onto Nodes.
- Per-site components (storage, metalman) node-select on **either** key so they can schedule before an upgraded net has converged all Nodes onto the canonical label (see `internal/operator/reconciler.go` `deprecatedSiteLabelKey` and the reaper in `internal/operator/migrate.go`).

Follow-up: in a future release, once all net controllers have converged Nodes to `unbounded-cloud.io/site`, remove the deprecated write and the dual-key fallback/affinity term.

Context: split out of the umbrella `unbounded-system` integration PR #388.

Start in internal/operator/reconciler.go at deprecatedSiteLabelKey, then trace the dual label write and the dual-key fallback/affinity term used by the per-site components. Read the reaper in internal/operator/migrate.go to understand the convergence condition. Done means removing the deprecated write and fallback once Nodes use the canonical label.

Written by the indexing model from the issue text.