remove deprecated ioutil references

[RohitKochhar]

This PR fixes the use of the now depracted ioutil package, replacing functions it provided with the according os or io functions.

Checklist

• I have signed the CLA
• I have updated/added any relevant documentation (not needed)

Description

What's the goal of this PR?

To remove any references to ioutil while ensuring the functionality that it provided is still in place with the newer alternatives.

What changes did you make?

Changed all references as follows:

• ioutil.TempFile -> os.CreateTemp
• ioutil.ReadFile -> os.ReadFile
• ioutil.ReadAll -> io.ReadAll
• ioutil.WriteFile -> os.WriteFile

Also changed a capitalized fmt.Errorf message as per Go convention.

What alternative solution should we consider, if any?

• Since the library is deprecated, if this change is omitted, there will eventually be issues with compatibility with newer versions of go.

[sudermanjr]

Thanks for the PR! LGTM, but we'll need a codeowner review.