[pkg/entrypoint] remove ioutil for new go version

[my-git9]

Changes

[pkg/entrypoint] remove ioutil for new go version
Ref: #5874

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Release Notes

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[tekton-robot]

Hi @my-git9. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[wlynch]

/ok-to-test

[wlynch]

/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wlynch

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[JeromeJu]

/kind cleanup