Replace deprecated `ioutil` package

[bryantbiggs]

What this PR does / why we need it:

• Replace deprecated ioutil package; this was deprecated back in go 1.16 https://github.com/golang/go/issues/42026

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #

[k8s-ci-robot]

Hi @bryantbiggs. Thanks for your PR.

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[nnmin-aws]

/ok-to-test

[bryantbiggs]

/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: bryantbiggs, jyotimahapatra

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/aws-iam-authenticator/blob/master/OWNERS)~~ [jyotimahapatra] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[nnmin-aws]

/lgtm

[nnmin-aws]

/merge