lint: remove io/ioutil

[wafuwafu13]

Description of changes:
io/ioutil is deprecated from Go 1.16.
If there is a problem with this commit 9f6aa76, I close this PR.
I know that lambda supports Go1.x, but I am not sure if I can remove Go1.15 or lower in CI.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #496 (d7004f6) into main (6645426) will not change coverage.
The diff coverage is 71.42%.

@@           Coverage Diff           @@
##             main     #496   +/-   ##
=======================================
  Coverage   71.18%   71.18%           
=======================================
  Files          21       21           
  Lines        1253     1253           
=======================================
  Hits          892      892           
  Misses        295      295           
  Partials       66       66           

Impacted Files	Coverage Δ
lambda/extensions_api_client.go	0.00% <0.00%> (ø)
cfn/response.go	69.44% <100.00%> (ø)
cmd/build-lambda-zip/main.go	34.66% <100.00%> (ø)
lambda/handler.go	95.58% <100.00%> (ø)
lambda/runtime_api_client.go	73.13% <100.00%> (ø)

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[bmoffatt]

Keeping lower compiler versions supported hasn't been an issue so far, so I've not had a reason to change it. Aside from the linter, i'm not aware of any downsides to continuing to use the ioutil -> io alias with newer compiler versions.

That said, I'm OK leaving the PR open, as I might need to drop 1.13-1.15 from CI anyways to merge #499

[bmoffatt]

Closing, decided to keep the tests running on old versions for now.