text: Title: [k8s] Raise socat/netcat dependency requirements in `sky check`
  Body: Currently we ask users to install netcat/socat when they run `sky launch`. Worse, if neither of the dependencies is installed, our error messages do not tell the user that both need to be installed, making it a sequential process:
  ```
  sky check kubernetes
  # Passes
  sky launch -c test --cloud k8s -- echo hi
  # Asks me to install socat
  # I run brew install socat and try again:
  sky launch -c test --cloud k8s -- echo hi
  # Now asks me to install netcat
  # Works after netcat is installed
  ```
  
  We should have one command to install both if neither is installed, and move it to `sky check` since it is a hard requirement. 