Rename `gotk-system` namespace to `flux-system`

.github/workflows/bootstrap.yaml

.github/workflows/e2e.yaml

cmd/gotk/create_alert.go

cmd/gotk/install.go

cmd/gotk/main.go

cmd/gotk/uninstall.go

docs/cmd/gotk.md

docs/cmd/gotk_bootstrap.md

docs/cmd/gotk_bootstrap_github.md

docs/cmd/gotk_bootstrap_gitlab.md

docs/cmd/gotk_check.md

docs/cmd/gotk_completion.md

docs/cmd/gotk_completion_bash.md

docs/cmd/gotk_completion_fish.md

docs/cmd/gotk_completion_powershell.md

docs/cmd/gotk_completion_zsh.md

docs/cmd/gotk_create.md

docs/cmd/gotk_create_alert-provider.md

docs/cmd/gotk_create_alert.md

docs/cmd/gotk_create_helmrelease.md