Resync mcp_server_enabled for stale root groups

What does this MR do and why?

The BackfillMcpServerEnabled background migration captured a point-in-time snapshot. Groups that changed duo_features_enabled after the backfill finished were left with a stale mcp_server_enabled value (see gitlab-com/request-for-help#4926).

This post-migration corrects all root groups where mcp_server_enabled disagrees with the formula (duo_features_enabled AND experiment_features_enabled). The original QueueBackfillMcpServerEnabled is made no-op since this migration supersedes it for fresh environments.

This change requires !240678 (merged) to merged first to ensure all new changes are synced.

References

gitlab-com/request-for-help#4926

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