Automatic merge from submit-queue

Allow NetworkPolicy.spec updates

ValidateNetworkPolicyUpdate currently prohibits changes to `spec` in an existing NetworkPolicy. We were going to fix this for 1.7 but I forgot to submit this PR after the main PR merged. Too late for 1.7? @thockin @caseydavenport @cmluciano 

This only changes networking.NetworkPolicy validation at the moment... Should I change extensions.NetworkPolicy validation too?

Fixes #35911

We should add a test to the e2e NetworkPolicy test for this too if this is going to merge.

**Release note**:
```release-note
As part of the NetworkPolicy "v1" changes, it is also now
possible to update the spec field of an existing
NetworkPolicy. (Previously you had to delete and recreate a
NetworkPolicy if you wanted to change it.)
```