docs,pkg: use new well-known region label

This commit updates the well-known label to determine the region of the
node to topology.kubernetes.io/region, which is the new standard as
defined by the Kubernetes documentation, now that
failure-domain.beta.kubernetes.io/region has been deprecated.

pkg/k8s/backend.go

@@ -58,7 +58,7 @@ const (
 	locationAnnotationKey        = "kilo.squat.ai/location"
 	wireGuardIPAnnotationKey     = "kilo.squat.ai/wireguard-ip"
 
-	regionLabelKey  = "failure-domain.beta.kubernetes.io/region"
+	regionLabelKey  = "topology.kubernetes.io/region"
 	jsonPatchSlash  = "~1"
 	jsonRemovePatch = `{"op": "remove", "path": "%s"}`
 )