Remove LegacyGitHubClient usage from Gitea Importer

What does this MR do?

This MR removes usage of legacy github importer from Gitea importer, with an end goal of removing the legacy code altogether (in the follow up MRs). It replaces legacy code with current GitHub Parallel Importer, which should not only bring consistency between two importers but also speed Gitea imports up (since that was the original reason why Parallel Importer was introduced).

Mentions #225211 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team