Unnecessary use of GitHub API for management cluster Flux bootstrap

Current instructions for GitHub Flux bootstrap requires the use of a GitHub PAT. This is because the bootstrap is being performed using the GitHub API i.e. using `flux bootstrap github`.

This can be avoided by using the Git protocol with the existing ssh key instead of the GitHub API e.g.
```
flux bootstrap git \
--url=ssh://git@github.com/rizblie/gitops-system.git \
--private-key-file=/home/ubuntu/.ssh/gitops \
--components-extra=image-reflector-controller,image-automation-controller \
--namespace=flux-system \
--branch=main \
--path=clusters/mgmt
```

This is a more generic solution as it does not rely on a GitHub-specific API and can be used with other Git hosting services.