Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Mark Flexvolume as GA

Flex volume became GA from release 1.8 onwards. This PR fixes the comments to reflect it.

Fixes #56920 

**Special notes for your reviewer**:

**Release note**:
```release-note
Flexvolume feature has graduated to GA.
```