[skip changelog] Updated the deprecated ioutil dependency

[zachary-walters]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

This change updates the codebase to use alternatives to the ioutil dependency. ioutil has been deprecated since go v1.16.

What is the current behavior?

The current behavior is unchanged.

What is the new behavior?

There is no new behavior

Does this PR introduce a breaking change, and is titled accordingly?

No

Other information

Please see go's official statement about ioutil's deprecation in v1.16: https://go.dev/doc/go1.16#ioutil

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Base: 36.56% // Head: 36.64% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (6715f39) compared to base (4b70e02).
Patch coverage: 35.29% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2054      +/-   ##
==========================================
+ Coverage   36.56%   36.64%   +0.07%     
==========================================
  Files         228      228              
  Lines       19376    19405      +29     
==========================================
+ Hits         7084     7110      +26     
  Misses      11463    11463              
- Partials      829      832       +3     

Flag	Coverage Δ
unit	36.64% <35.29%> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
arduino/resources/checksums.go	43.56% <0.00%> (ø)
internal/cli/cli.go	0.00% <0.00%> (ø)
legacy/builder/utils/utils.go	54.63% <0.00%> (-0.77%)	⬇️
legacy/builder/gohasissues/go_has_issues.go	50.00% <66.66%> (-0.77%)	⬇️
commands/lib/search.go	88.15% <0.00%> (-3.95%)	⬇️
arduino/cores/packagemanager/package_manager.go	68.19% <0.00%> (+2.36%)	⬆️
arduino/cores/packagemanager/download.go	34.61% <0.00%> (+5.12%)	⬆️

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Requested change has been made. Thanks!

[cmaglie]

HI @zachary-walters!

I've added a couple of nitpicks, but for the rest LGTM, we can merge it as soon as you fix them.
Thanks!