message: change openapi version to v15
diff: @@ -14,7 +14,7 @@ To use the openapi generated types:
  ```toml
  [dependencies]
  kube = { version = "0.15.0", features = ["openapi"] }
  -k8s-openapi = { version = "0.5.0", features = ["v1_14"] }
  +k8s-openapi = { version = "0.5.0", features = ["v1_15"] }
  ```
  otherwise:
  