chore: Replace deprecated ioutil in CLI code

[jannfis]

The io/ioutil package has been deprecated in Go 1.16.

This is part 1/x of getting rid of io/ioutil in our codebase, replacing it by appropriate other methods from Go's standard library.

Focus: CLI code.

Signed-off-by: jannfis jann@mistrust.net

Note on DCO:

If the DCO action in the integration test fails, one or more of your commits are not signed off. Please click on the Details link next to the DCO action for instructions on how to resolve this.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• Optional. My organization is added to USERS.md.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).

[codecov]

Codecov Report

Merging #9846 (1d9d991) into master (2a3c692) will not change coverage.
The diff coverage is 13.63%.

@@           Coverage Diff           @@
##           master    #9846   +/-   ##
=======================================
  Coverage   45.75%   45.75%           
=======================================
  Files         227      227           
  Lines       26961    26961           
=======================================
  Hits        12337    12337           
  Misses      12943    12943           
  Partials     1681     1681           

Impacted Files	Coverage Δ
cmd/argocd/commands/admin/app.go	30.37% <0.00%> (ø)
cmd/argocd/commands/admin/backup.go	0.00% <0.00%> (ø)
cmd/argocd/commands/admin/repo.go	0.00% <0.00%> (ø)
cmd/argocd/commands/context.go	15.18% <0.00%> (ø)
cmd/argocd/commands/gpg.go	0.00% <0.00%> (ø)
cmd/argocd/commands/repo.go	0.00% <0.00%> (ø)
cmd/argocd/commands/repocreds.go	0.00% <0.00%> (ø)
cmd/argocd/commands/admin/settings.go	68.60% <66.66%> (ø)
cmd/argocd/commands/admin/project_allowlist.go	30.95% <100.00%> (ø)
util/settings/settings.go	48.22% <0.00%> (ø)

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[crenshaw-dev]

lgtm! @jannfis can you sort the conflicts?