Move Import::GithubService#repository_size_limit to EE

What does this MR do and why?

We need to move the logic of repository_size_limit in Import::GithubService to EE because:

1. Repository size limit is part of EE features: Docs
2. This logic is already defined in the model, and we should point it by using target_namespace.actual_repository_size_limit

References

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• Issue: #502222 (closed)

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How to set up and validate locally

The tests should cover this