cmd/gaiad/cmd: replace deprecated ioutil with os.WriteFile (backport #1660)

[mergify]

This is an automatic backport of pull request #1660 done by Mergify.
Cherry-pick of 6ac473a has failed:

On branch mergify/bp/release/v7.0.x/pr-1660
Your branch is up to date with 'origin/release/v7.0.x'.

You are currently cherry-picking commit 6ac473a.
  (fix conflicts and run "git cherry-pick --continue")
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  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cmd/gaiad/cmd/testnet.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release/v7.0.x@d0884c2). Click here to learn what that means.
The diff coverage is n/a.

@@                Coverage Diff                @@
##             release/v7.0.x    #1661   +/-   ##
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