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 from ioutil to io/os:

• Replaced ioutil.ReadAll with io.ReadAll in imagepolicy.go and imagepolicy_test.go for reading configuration data from readers. [1] [2] [3]
• Replaced ioutil.WriteFile with os.WriteFile in audit.go for writing audit policy files.

Cleanup of deprecated imports:

• Removed all ioutil import statements from imagepolicy.go, imagepolicy_test.go, and audit.go. [1] [2] [3]

[openshift-ci]

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[ibihim]

/lgtm

[openshift-ci]

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