text: Title: modifying namespace spec.finalizers via PATCH should return a validation error
  Body: **What happened**:
  
  PATCH `{"spec":{"finalizers":null}}` doesn't clear the finalizers and also doesn't return an error.
  
  **What you expected to happen**: It should return an error (use the /finalize subresource).
  
  **How to reproduce it (as minimally and precisely as possible)**: see #77086
  