Remove octokit gem

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This issue is part of the &16949 epic. See that epic's description for more context.

About

We want to remove the octokit gem which is used by the GitHub importer and Gitea Importer.

The work includes:

• Remove the octokit gem, middleware and patches.
• Remove faraday-multipart which was added for octokit - being verified in #521811 (comment 2371922440).
• Write a small HTTP client to interact with the GitHub API. All HTTP calls must use Gitlab::HTTP. With special handling of rate limits. Refactor Gitlab::GithubImport::Client. The API interactions must be neat and maintainable. Refactoring any error handling of Octokit errors. Support Pagination and GitHub Enterprise.
• Similary, refactor Gitlab::LegacyGithubImport::Client used by Gitea

AppSec review

On every MR, the Application Security Engineer stable counterpart for Import and Integrate (see this page - @ameyadarshan at time of writing) must be asked to review. The review is not blocking, so can be conducted after the MR merges.