Change `fluxcd/toolkit` to `fluxcd/flux2`

CONTRIBUTING.md

README.md

cmd/flux/bootstrap.go

cmd/flux/bootstrap_github.go

cmd/flux/bootstrap_gitlab.go

cmd/flux/check.go

cmd/flux/create_alert.go

cmd/flux/create_alertprovider.go

cmd/flux/create_helmrelease.go

cmd/flux/create_kustomization.go

cmd/flux/create_receiver.go

cmd/flux/create_source_bucket.go

cmd/flux/create_source_git.go

cmd/flux/create_source_helm.go

cmd/flux/create_tenant.go

cmd/flux/delete_alert.go

cmd/flux/delete_alertprovider.go

cmd/flux/delete_helmrelease.go

cmd/flux/delete_kustomization.go

cmd/flux/delete_receiver.go