refactor: move from io/ioutil to io and os packages

[Juneezee]

The io/ioutil package has been deprecated in Go 1.16 (See https://pkg.go.dev/io/ioutil). This PR replaces the existing io/ioutil functions with their new definitions in io and os packages.

• ioutil.Discard => io.Discard
• ioutil.NopCloser => io.NopCloser
• ioutil.ReadAll => io.ReadAll
• ioutil.ReadDir => os.ReadDir
• ioutil.ReadFile => os.ReadFile
• ioutil.TempDir => os.MkdirTemp
• ioutil.TempFile => os.CreateTemp
• ioutil.WriteFile => os.WriteFile

ioutil.TempDir in *_test.go files can be replaced with t.TempDir from the testing package. The directory created by t.TempDir is automatically removed when the test and all its subtests complete.

Reference: https://pkg.go.dev/testing#T.TempDir

[cyberb]

Thank you very much!

Currently integration tests do not function on PRs as credentials for test domains are not visible on them, probably it is time for me to introduce test docker image for Syncloud DDNS service to use. https://github.com/syncloud/platform/issues/662

Still meging this of cause.