cleanup: remove io/ioutil

[my-git9]

The package about io/ioutil has been deprecated from go 1.16: https://go.dev/doc/go1.16#ioutil
We should remove it for go version 1.18 which used by tekton/pipeline.
We can clean up by directory in batches, like:#5864

[JeromeJu]

Thanks @my-git9 for your help! And thanks for dividing up the work within directories. For now the ./cmd/entrypoint is taken care of, we could use more help within other directories.

/kind cleanup

[tekton-robot]

Issues go stale after 90d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale with a justification.
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 with a justification.
If this issue should be exempted, mark the issue as frozen with /lifecycle frozen with a justification.

/lifecycle stale

Send feedback to tektoncd/plumbing.

[lbernick]

I don't see any more usage of "io/ioutil" outside of our vendor directory so I'm going to go ahead and close-- thanks @my-git9 for your contributions! Feel free to reopen if there's something I missed.