Migrate use of ioutil.ReadFile (deprecated) to os.ReadFile

[buzzsurfr]

go's parser gives a warning about 42 references to ioutil.ReadFile in the repo, stating:

"io/ioutil" has been deprecated since Go 1.16: As of Go 1.16, the same functionality is now provided by package io or package os, and those implementations should be preferred in new code. See the specific function documentation for details.  (SA1019)

An example offending line is interop/client/client.go#L156

b, err := ioutil.ReadFile(*caFile)

According to the message, this should be replaced with os.ReadFile.

What version of gRPC are you using?

1.51

What version of Go are you using (go version)?

go1.19.4 darwin/arm64

What operating system (Linux, Windows, …) and version?

MacOS, but applies to all

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Created via thread in #5882 (comment)

/assign @buzzsurfr

[easwars]

Thanks for filing this issue. And thanks for volunteering to work on this.

[buzzsurfr]

After further review, the entire package io/ioutil has been deprecated. I submitted a commit on the PR for just ReadFile but will submit another commit for the rest of the package use.