chore: remove deprecated io/ioutil

cmd/flux/bootstrap.go

cmd/flux/bootstrap_git.go

cmd/flux/bootstrap_github.go

cmd/flux/bootstrap_gitlab.go

cmd/flux/create_helmrelease.go

cmd/flux/create_source_bucket.go

cmd/flux/create_source_git.go

cmd/flux/create_source_helm.go

cmd/flux/install.go

internal/bootstrap/bootstrap_plain_git.go

internal/utils/utils.go

internal/utils/utils_test.go

pkg/manifestgen/install/install.go

pkg/manifestgen/install/manifests.go

pkg/manifestgen/kustomization/kustomization.go

pkg/manifestgen/manifest.go

pkg/manifestgen/sourcesecret/sourcesecret.go

pkg/manifestgen/sourcesecret/sourcesecret_test.go