refactor: move from io/ioutil to io and os packages

[Juneezee]

The io/ioutil package has been deprecated in Go 1.16 (See https://pkg.go.dev/io/ioutil). This PR replaces the existing io/ioutil functions with their new definitions in io and os packages.

• ioutil.Discard => io.Discard
• ioutil.NopCloser => io.NopCloser
• ioutil.ReadAll => io.ReadAll
• ioutil.ReadDir => os.ReadDir (returns a slice of os.DirEntry rather than a slice of fs.FileInfo)
• ioutil.ReadFile => os.ReadFile
• ioutil.TempDir => os.MkdirTemp
• ioutil.TempFile => os.CreateTemp
• ioutil.WriteFile => os.WriteFile

[heronrs]

Thanks for the PR @Juneezee ! We'll merge as soon as we can

[codecov[bot]]

Codecov Report

Base: 36.23% // Head: 36.23% // No change to project coverage :thumbsup:

Coverage data is based on head (6620b80) compared to base (1d65f60). Patch coverage: 43.33% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3582   +/-   ##
=======================================
  Coverage   36.23%   36.23%           
=======================================
  Files         168      168           
  Lines       18413    18413           
=======================================
  Hits         6672     6672           
  Misses      10608    10608           
  Partials     1133     1133           

Impacted Files	Coverage Δ
pkg/api/controllers.go	62.61% <ø> (ø)
pkg/api/routes.go	100.00% <ø> (ø)
pkg/cli/auth.go	12.19% <0.00%> (ø)
pkg/cli/env.go	57.60% <0.00%> (ø)
pkg/cli/update.go	18.51% <0.00%> (ø)
pkg/generate/method.go	32.43% <0.00%> (ø)
pkg/helpers/aws.go	37.08% <0.00%> (ø)
pkg/helpers/file.go	0.00% <0.00%> (ø)
pkg/helpers/linux.go	0.00% <0.00%> (ø)
pkg/helpers/tar.go	0.00% <0.00%> (ø)
... and 24 more

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