Replace Legacy Github Importer with Parallel Importer in Gitea Importer
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Similar to what's been done for GitHub Importer https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/import_sources.rb#L14
Replace Legacy Github Importer with Gitlab::GithubImport::ParallelImporter in order to bring two importers closer together, since Gitea API is github compatible and Octokit client can be used to fetch data from Gitea when importing into GitLab. Additional benefits are distributed import via multiple sidekiq jobs which makes import much faster, bulk insert of relations (issues, labels, etc) also has positive on performance.
The work has been started here !37934 (closed)
Additionally, consider #232982 when implementing this to, while reusing a lot of representation classes and parallel importer, perhaps define separate github/gitea stages to allow better maintainability and extensibility of each importers importing flow.