✨ clusterctl: Add support for valid GitHub URLs

[HomayoonAlimohammadi]

What this PR does / why we need it:
Following this already closed issue, URLs in the clusterctl config are invalid and do not point to any resource. This PR adds the support for having valid URLs while also keeping backwards compatibility by supporting the old (invalid) format.

Which issue(s) this PR fixes:
In #2830, the invalid URL part was seemingly ignored in the mentioned issue.

/area clusterctl

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[k8s-ci-robot]

Hi @HomayoonAlimohammadi. Thanks for your PR.

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