Automatic merge from submit-queue

kubeadm: Rename FeatureFlags to FeatureGates

**What this PR does / why we need it**:

Automatic rename from `FeatureFlags` to `FeatureGates`, as I noticed that's the real name for this feature. This is for consistency in the API and generally in the code.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```
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