Replace deprecated use of ioutil with io and os equivalents

[atombrella]

I wonder why this was allowed to survive this long. Anyway, replace in VSCode for the os and io functions was my solution.

https://pkg.go.dev/io/ioutil

I can't add the no-changelog-needed label, but this change is transparent to the user, and only serves to update the code quality.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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[github-actions]

Changelog Warning

Currently this PR would target a v1.15 release. Please add a changelog entry for in the .changes/v1.15 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.

[jbardin]

Hi @atombrella,

While these are deprecated, they are still functionally equivalent and not scheduled to be removed. We tend to not accept PRs that that touch so many files with no functional changes to as to not conflict with work in progress, and opt to replace the usage in a more piecemeal fashion as we work on individual packages.

[atombrella]

@jbardin 👍 I'll close this PR.