Change copyright to Flux authors

cmd/gotk/bootstrap.go

cmd/gotk/bootstrap_github.go

cmd/gotk/bootstrap_gitlab.go

cmd/gotk/check.go

cmd/gotk/completion.go

cmd/gotk/completion_bash.go

cmd/gotk/completion_fish.go

cmd/gotk/completion_powershell.go

cmd/gotk/completion_zsh.go

cmd/gotk/create.go

cmd/gotk/create_alert.go

cmd/gotk/create_alertprovider.go

cmd/gotk/create_helmrelease.go

cmd/gotk/create_kustomization.go

cmd/gotk/create_receiver.go

cmd/gotk/create_source.go

cmd/gotk/create_source_bucket.go

cmd/gotk/create_source_git.go

cmd/gotk/create_source_helm.go

cmd/gotk/create_tenant.go