Update k8s.io/kube-openapi to v0.0.0-20260721132016-d427ff9ee9ad

Picks up:
- kubernetes/kube-openapi#625: update the go-openapi/swag family to v0.27.1
- kubernetes/kube-openapi#622: propagate nullable field between SMD and OpenAPI schemas

Transitive dependency changes:
- github.com/go-openapi/swag (and submodules) v0.25.4 -> v0.27.1
- github.com/go-openapi/jsonpointer v0.22.4 -> v1.0.0
- github.com/go-openapi/jsonreference v0.21.4 -> v1.0.0
- github.com/go-openapi/swag/jsonname removed (package now provided by jsonpointer)
- github.com/go-openapi/swag/pools added

No changes to hack/unwanted-dependencies.json needed.

Kubernetes-commit: 0706a3bb8948385550b71e5c01f30b30f7041200