Remove use of Legacy GitHub Client code from Gitea Importer & Other places
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=235764) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=235764) </details> <!--IssueSummary end--> 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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