Remove github_user_mapping feature flag (!216778) · Merge requests · GitLab.org / GitLab

What does this MR do and why?

Removes the github_user_mapping feature flag definition from the codebase to ensure new migrations always use the new user mapping.

As per the comment in #510963, we need to remove the feature flag definition in a separate MR from removing the old user mapping code. This prevents issues during in-flight migrations on self-managed instances where the feature flag was manually disabled.

The old user mapping code (add_complementary_review_note! method) is intentionally NOT removed in this MR. It will be removed in a separate MR to allow for safer rollout across versions.

References

Related to #510963

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