Remove use of Legacy GitHub Client code from Gitea Importer & Other places
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Similar to https://gitlab.com/gitlab-org/gitlab/-/issues/225211
Replace Legacy Github Client with newer `Gitlab::GithubImport::Client` since it is legacy in order to make github / gitea importers more consistent and in the future remove legacy github client altogether.
https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/legacy_github_import/importer.rb#L39
Some of the work has been started in:
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38671
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37934
Consider skipping this step completely and replacing Legacy Github Importer with Parallel Importer for Gitea Importer https://gitlab.com/gitlab-org/gitlab/-/issues/235770
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