Update generator OWNERS
Update generator OWNERS

I reviewed PRs over the last year:
- nikhita has done consistent review on almost every change
  that touches this code, added a number of substantive
  changes herself, and is a contribex tech lead
- pruning grodrigues3 due to lack of activity in the past year

kubernetes/community,pwittrock/community,kubernetes/community,kubernetes/community,pwittrock/community