Automatic merge from submit-queue (batch tested with PRs 37032, 38119, 38186, 38200, 38139)

etcd2: remove unnecessary PrevValue in SetOption

ref: https://github.com/kubernetes/kubernetes/issues/37994

Summary:
- PrevValue is set in HTTP header, and large value (>1MB) could exceed check limit
- We don't need PrevValue indeed since we already use PrevIndex in SetOptions and each PrevIndex corresponds to each PrevValue.

I don't really think we need extra tests for this. There is already test for GuaranteedUpdate covering its use cases.