Migrate flex adapter from drivers

[msau42]

We're going to deprecate the drivers repo.

https://github.com/kubernetes-csi/drivers/tree/master/pkg/flexadapter

[msau42]

/assign @chakri-nelluri @shilpamayanna

[k8s-ci-robot]

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In response to this:

/assign @chakri-nelluri @shilpamayanna

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[fejta-bot]

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[cofyc]

/remove-lifecycle stale

[fejta-bot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
Stale issues rot after an additional 30d of inactivity and eventually close.

If this issue is safe to close now please do so with /close.

Send feedback to sig-testing, kubernetes/test-infra and/or fejta.
/lifecycle stale