Migrate away from deprecated ioutil

[sebrandon1]

ioutil has been deprecated since Go 1.16: https://go.dev/doc/go1.16#ioutil

Tracking issue: redhat-best-practices-for-k8s/telco-bot#52

Migration away from ioutil:

• Replaced ioutil.TempFile with os.CreateTemp for creating temporary files in build_controller_test.go.
• Updated ioutil.NopCloser to io.NopCloser and ioutil.ReadAll to io.ReadAll in imagestream_controller_test.go and templateinstance_controller_test.go. [1] [2] [3]

Cleanup of imports:

• Removed unused ioutil imports and added io where necessary in test files. [1] [2] [3]

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