refactor: remove io/ioutil dependency

[ginglis13]

Description

io/ioutil has been deprecated since Go 1.16
https://pkg.go.dev/io/ioutil

adding hacktoberfest label would be much appreciated :)

Issues Resolved

n/a

Check List

• [n/a] New functionality includes testing.
  • All tests pass
• [n/a] New functionality has been documented.
  • [n/a] New functionality has documentation added
• Commits are signed as per the DCO using --signoff

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[codecov]

Codecov Report

Merging #86 (054d7ac) into main (c53ff19) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##             main      #86   +/-   ##
=======================================
  Coverage   64.60%   64.60%           
=======================================
  Files          32       32           
  Lines        2328     2328           
=======================================
  Hits         1504     1504           
  Misses        716      716           
  Partials      108      108           

Flag	Coverage Δ
opensearch-cli	64.60% <83.33%> (ø)

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

Files	Coverage Δ
controller/config/config.go	67.85% <100.00%> (ø)
handler/ad/ad.go	87.73% <100.00%> (ø)
mapper/platform/platform.go	97.16% <100.00%> (ø)
gateway/gateway.go	34.93% <50.00%> (ø)

[VachaShah]

Thank you @ginglis13!