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 os/io functions:

• Replaced ioutil.ReadAll with io.ReadAll in pkg/bootstrapinplace/bootstrapinplace.go. [1] [2]
• Replaced ioutil.ReadFile with os.ReadFile in pkg/start/asset.go and pkg/start/bootstrap_test.go. [1] [2]
• Replaced ioutil.TempDir with os.MkdirTemp in pkg/start/bootstrap_test.go.
• Replaced ioutil.WriteFile with os.WriteFile in pkg/start/bootstrap_test.go. [1] [2]
• Removed unused ioutil imports from affected files. [1] [2]

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