Replace legacy github client in CreateGithubWebhookWorker

What does this MR do?

This MR replaces Gitlab::LegacyGithubImport::Client in CreateGithubWebhookWorker with Gitlab::GithubImport::Client in order to remove legacy code in the future MR as part of #225211 (closed)

Change is behind remove_legacy_github_client feature flag, which is already enabled on GitLab.com as this feature flag is used in multiple places (e.g. github controller has similar feature flag check).

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