Backport of 'Add rate limit handling to imports from GHES' to 19.4

What does this MR do and why?

Backport of !254281 (merged) to the 19-4-stable-ee stable branch.

The GitHub import client now waits for rate limits to expire when importing from GitHub Enterprise Server (GHES). Previously the importer assumed rate limiting was never enabled on GHES instances (it is only disabled by default), so imports from rate-limited instances exhausted worker retries and failed to import all project resources. The effect was more pronounced with lower rate limits.

Original MR: !254281 (merged) Cherry-picked squash commit: e3e26693

Why this warrants a backport

Despite the typefeature label on the original issue, this change was requested by a customer because they were unable to migrate their repositories from GHES to GitLab due to strict rate limits on their GHES instance. Our importer assumed that all GHES instances were not rate limited, but that's not true. This change is arguably typebug, so I've labelled as such

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.