Remove use of LegacyGithubClient in GithubController

What does this MR do?

This MR removes use of LegacyGithubImport::Client in favour of GithubImport::Client in GitHubController with an end goal of removing the legacy code altogether (in the follow up MRs, since the legacy client is used in a few other places).

Changes in this MR remove dependency on legacy client and unblock development of #27370 (closed)

Mentions #225211 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

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
Edited by George Koltsov