chore: Drop deprecated io/ioutil

[shuheiktgw]

Removed deprecated io/ioutil from the source code. Thank you for your review!
Ref: fluxcd/flux2#1658

[codecov-commenter]

Codecov Report

Merging #964 (38d3ca1) into main (df459c5) will not change coverage.
The diff coverage is 52.94%.

@@           Coverage Diff           @@
##             main     #964   +/-   ##
=======================================
  Coverage   56.91%   56.91%           
=======================================
  Files          71       71           
  Lines        5861     5861           
=======================================
  Hits         3336     3336           
  Misses       2030     2030           
  Partials      495      495           

Impacted Files	Coverage Δ
pkg/loadtester/concord.go	29.03% <0.00%> (ø)
pkg/notifier/client.go	31.42% <0.00%> (ø)
pkg/loadtester/server.go	12.94% <14.28%> (ø)
pkg/controller/webhook.go	64.70% <100.00%> (ø)
pkg/loadtester/task_ngrinder.go	48.14% <100.00%> (ø)
pkg/metrics/providers/datadog.go	63.38% <100.00%> (ø)
pkg/metrics/providers/graphite.go	61.61% <100.00%> (ø)
pkg/metrics/providers/newrelic.go	59.37% <100.00%> (ø)
pkg/metrics/providers/prometheus.go	63.63% <100.00%> (ø)

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[stefanprodan]

LGTM

Thanks @shuheiktgw 🥇