Update github import known issues
What does this MR do and why?
This adds a known issue to the GitHub importer documentation. See #418800 (closed) for additional context
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.
Related to #418800 (closed)